Если не охота заморачиваться с маской для strptime то можно так.

from dateutil.parser import parse

text = 'Sun 10 May 2021 13:54:36 -0700'

print(parse(text))
# 2021-05-10 13:54:36-07:00
print(repr(parse(text)))
# datetime.datetime(2021, 5, 10, 13, 54, 36, tzinfo=tzoffset(None, -25200)

Но если прям очень захочется то:

from datetime import datetime

text = 'Sun 10 May 2021 13:54:36 -0700'

pattern = '%a %d %b %Y %X %z'

print(datetime.strptime(text, pattern))
print(repr(datetime.strptime(text, pattern)))

#2021-05-10 13:54:36-07:00
#datetime.datetime(2021, 5, 10, 13, 54, 36, tzinfo=datetime.timezone(datetime.timedelta(days=-1, seconds=61200)))

# UPD
from datetime import datetime

pattern = '%a %d %b %Y %X %z'

with open('data.txt', 'r', encoding='utf-8') as file:
    for line in file.readlines():
        print(datetime.strptime(line.strip(), pattern))
#2021-05-10 13:54:36-07:00
#2021-05-10 13:54:36+00:00
#2021-05-02 19:54:36+05:30
#2021-05-01 13:54:36+00:0

from datetime import datetime

pattern = '%a %d %b %Y %X %z'

with open('data.txt', 'r', encoding='utf-8') as file:
    dates = [
        *map(
            lambda x: datetime.strptime(x.strip(), pattern),
            filter(lambda x: bool(x.strip()), file.readlines())
        )
    ]

for f, s in zip(dates[::2], dates[1::2]):
    print(abs(int(f.timestamp() - s.timestamp())))

Datetime parsing patterns

Several additional directives not required by the C89 standard are included for convenience. These parameters all correspond to ISO 8601 date values.