Открытие нового окна через кнопку

НИКОГДА НЕ ИЗМЕНЯЙТЕ код, сгенерированный Qt Designer, НИКОГДА.
Создайте другой класс, который наследуется от соответствующего виджета, и используйте созданный класс для его заполнения.

q1419969_main.py

from PyQt5 import QtCore, QtGui, QtWidgets
from q1419969_test import Ui_Test


class Ui_Login(object):
    def setupUi(self, Login):
        Login.setObjectName("Login")
        Login.resize(1100, 600)
        Login.setStyleSheet("background-color: rgb(255, 255, 255);")
        self.centralwidget = QtWidgets.QWidget(Login)
        self.centralwidget.setObjectName("centralwidget")
        self.label = QtWidgets.QLabel(self.centralwidget)
        self.label.setGeometry(QtCore.QRect(280, 120, 581, 41))
        sizePolicy = QtWidgets.QSizePolicy(QtWidgets.QSizePolicy.Maximum, QtWidgets.QSizePolicy.Maximum)
        sizePolicy.setHorizontalStretch(0)
        sizePolicy.setVerticalStretch(0)
        sizePolicy.setHeightForWidth(self.label.sizePolicy().hasHeightForWidth())
        self.label.setSizePolicy(sizePolicy)
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(14)
        self.label.setFont(font)
        self.label.setStyleSheet("background-color: rgb(198, 212, 255);")
        self.label.setObjectName("label")
        self.pushButton = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton.setGeometry(QtCore.QRect(490, 440, 141, 71))
        sizePolicy = QtWidgets.QSizePolicy(QtWidgets.QSizePolicy.Fixed, QtWidgets.QSizePolicy.Fixed)
        sizePolicy.setHorizontalStretch(0)
        sizePolicy.setVerticalStretch(0)
        sizePolicy.setHeightForWidth(self.pushButton.sizePolicy().hasHeightForWidth())
        self.pushButton.setSizePolicy(sizePolicy)
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(14)
        self.pushButton.setFont(font)
        self.pushButton.setStyleSheet("background-color: rgb(198, 212, 255);")
        self.pushButton.setObjectName("pushButton")
        self.label_3 = QtWidgets.QLabel(self.centralwidget)
        self.label_3.setGeometry(QtCore.QRect(480, 310, 141, 31))
        sizePolicy = QtWidgets.QSizePolicy(QtWidgets.QSizePolicy.Maximum, QtWidgets.QSizePolicy.Maximum)
        sizePolicy.setHorizontalStretch(0)
        sizePolicy.setVerticalStretch(0)
        sizePolicy.setHeightForWidth(self.label_3.sizePolicy().hasHeightForWidth())
        self.label_3.setSizePolicy(sizePolicy)
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(14)
        self.label_3.setFont(font)
        self.label_3.setStyleSheet("background-color: rgb(198, 212, 255);")
        self.label_3.setObjectName("label_3")
        self.label_2 = QtWidgets.QLabel(self.centralwidget)
        self.label_2.setGeometry(QtCore.QRect(520, 190, 61, 41))
        sizePolicy = QtWidgets.QSizePolicy(QtWidgets.QSizePolicy.Maximum, QtWidgets.QSizePolicy.Maximum)
        sizePolicy.setHorizontalStretch(0)
        sizePolicy.setVerticalStretch(0)
        sizePolicy.setHeightForWidth(self.label_2.sizePolicy().hasHeightForWidth())
        self.label_2.setSizePolicy(sizePolicy)
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(14)
        self.label_2.setFont(font)
        self.label_2.setStyleSheet("background-color: rgb(198, 212, 255);")
        self.label_2.setObjectName("label_2")
        self.lineEdit_2 = QtWidgets.QLineEdit(self.centralwidget)
        self.lineEdit_2.setGeometry(QtCore.QRect(380, 260, 331, 31))
        sizePolicy = QtWidgets.QSizePolicy(QtWidgets.QSizePolicy.Fixed, QtWidgets.QSizePolicy.Fixed)
        sizePolicy.setHorizontalStretch(0)
        sizePolicy.setVerticalStretch(0)
        sizePolicy.setHeightForWidth(self.lineEdit_2.sizePolicy().hasHeightForWidth())
        self.lineEdit_2.setSizePolicy(sizePolicy)
        self.lineEdit_2.setObjectName("lineEdit_2")
        self.lineEdit_3 = QtWidgets.QLineEdit(self.centralwidget)
        self.lineEdit_3.setGeometry(QtCore.QRect(380, 360, 331, 31))
        sizePolicy = QtWidgets.QSizePolicy(QtWidgets.QSizePolicy.Fixed, QtWidgets.QSizePolicy.Fixed)
        sizePolicy.setHorizontalStretch(0)
        sizePolicy.setVerticalStretch(0)
        sizePolicy.setHeightForWidth(self.lineEdit_3.sizePolicy().hasHeightForWidth())
        self.lineEdit_3.setSizePolicy(sizePolicy)
        self.lineEdit_3.setObjectName("lineEdit_3")
        self.label_4 = QtWidgets.QLabel(self.centralwidget)
        self.label_4.setGeometry(QtCore.QRect(-4, -8, 1111, 811))
        self.label_4.setAutoFillBackground(False)
        self.label_4.setText("")
        self.label_4.setPixmap(QtGui.QPixmap("images2.jpg"))
        self.label_4.setScaledContents(True)
        self.label_4.setObjectName("label_4")
        self.label_4.raise_()
        self.label.raise_()
        self.pushButton.raise_()
        self.label_3.raise_()
        self.label_2.raise_()
        self.lineEdit_2.raise_()
        self.lineEdit_3.raise_()
        Login.setCentralWidget(self.centralwidget)

        self.retranslateUi(Login)
        QtCore.QMetaObject.connectSlotsByName(Login)

    def retranslateUi(self, Login):
        _translate = QtCore.QCoreApplication.translate
        Login.setWindowTitle(_translate("Login", "Авторизация"))
        self.label.setText(_translate("Login", "<html><head/><body><p align=\"center\"><span style=\" font-size:14pt;\">Для прохождения теста необходимо заполнить ФИО и номер группы</span></p></body></html>"))
        self.pushButton.setText(_translate("Login", "Продолжить"))
        self.label_3.setText(_translate("Login", "<html><head/><body><p align=\"center\"><span style=\" font-size:14pt;\">Номер группы</span></p></body></html>"))
        self.label_2.setText(_translate("Login", "<html><head/><body><p align=\"center\"><span style=\" font-size:14pt;\">ФИО</span></p></body></html>"))


class TestWindow(QtWidgets.QMainWindow, Ui_Test):
    def __init__(self):
        super(TestWindow, self).__init__()
        self.setupUi(self) 


class MainWindow(QtWidgets.QMainWindow, Ui_Login):
    def __init__(self):
        super(MainWindow, self).__init__()
        self.setupUi(self) 
        
        self.pushButton.clicked.connect(self.opentest) 

    def opentest(self):
        self.test = TestWindow()
        self.test.show()
        self.hide()
        
        
if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    w = MainWindow()
    w.show()
    sys.exit(app.exec_())

q1419969_test.py

from PyQt5 import QtCore, QtGui, QtWidgets


class Ui_Test(object):
    def setupUi(self, Test):
        Test.setObjectName("Test")
        Test.resize(1100, 800)
        Test.setStyleSheet("background-color: rgb(255, 255, 255);")
        self.centralwidget = QtWidgets.QWidget(Test)
        self.centralwidget.setObjectName("centralwidget")
        Test.setCentralWidget(self.centralwidget)
        self.statusbar = QtWidgets.QStatusBar(Test)
        self.statusbar.setObjectName("statusbar")
        Test.setStatusBar(self.statusbar)
        self.menubar = QtWidgets.QMenuBar(Test)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 1100, 21))
        self.menubar.setObjectName("menubar")
        Test.setMenuBar(self.menubar)

        self.retranslateUi(Test)
        QtCore.QMetaObject.connectSlotsByName(Test)

    def retranslateUi(self, Test):
        _translate = QtCore.QCoreApplication.translate
        Test.setWindowTitle(_translate("Test", "Тест"))