datetime.datetime.utcnow() should not have tzinfo

[Pilen]

Hi, I find this library very neat and have started using it in multiple tests.
But there is one issue:

In [1]: import datetime

In [2]: import time_machine

In [3]: @time_machine.travel("2020-07-28 13:15:30.478705")
   ...: def foo():
   ...:     return datetime.datetime.utcnow()
   ...:

In [4]: datetime.datetime.utcnow()
Out[4]: datetime.datetime(2020, 7, 31, 13, 46, 25, 828701)

In [5]: foo()
Out[5]: datetime.datetime(2020, 7, 28, 13, 15, 30, 478705, tzinfo=datetime.timezone.utc)

time_machine adds a timezone on datetime instances created via .utcnow() but this is wrong according to python.
It even specifies it directly in the documentation https://docs.python.org/3/library/datetime.html#datetime.datetime.utcnow

Return the current UTC date and time, with tzinfo None.

This change in the behaviour of the datetime library can cause changes in behaviour of the tested code.
Eg. when serialized to a string it adds "+00:00"

[bbbart]

I second this. I have a test testing code including pytz.utc.localize(datetime.utcnow()...) which now raises a ValueError during time travel, making it impossible to use this library in that particular test.

[adamchainz]

Thanks for the report. I've released the fix in version 1.2.1.