Support for tz offset in string datetime spec

[danpalmer]

freezegun allows a datetime spec string to encode a timezone offset like this:

>>> with freeze_time('2020-04-15 12:00 +1'):
...     print(repr(datetime.datetime.now(tz=dateutil.tz.UTC)))
FakeDatetime(2020, 4, 15, 11, 0, tzinfo=tzutc())

This produces a datetime with the offset given in the string, meaning that now will return datetimes relative to this.

tz_offset behaviour differs from offset string

This behaviour differs to the tz_offset parameter that fake_time takes, as with freezegun it's essentially pre-baked into the time, whereas fake_time then treats it as additive on top of the time spec.

Depending on how this is implemented/documented, this might be a breaking change. I'm not sure how much this library aims to maintain compatibility with freezegun. From reading the documentation I had assumed it was API compatible for the public API, but issues like this ended up causing issues in porting our codebase.

I'm not reporting this as a separate issue because it's not strictly an issue, but more just a gotcha for working around the main issue here – it bit us when we tried to use it as a workaround.

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Note: to implement support for this will likely require addressing #59.