Bugfix datetime serialization utc zero offset

[stefan-jozefowicz-bayer]

Proposed Changes:

For inclusion in changelog (if applicable):

Not intended for changelog:

1. Change how timezone aware datetimes with UTC tz are converted to datetime string

Diff of User Interface

Old behavior:
passing a timezone aware datetime from the UTC zone to some of the request constructors resulted in malformed datetime strings, e.g: 2022-06-01T01:00:00+00:00Z - i.e. strings that have BOTH +00:00 and Z at the end.

At the root of it was value.utcoffset() >> timedelta(hours=0) evaluating as False even though there is a defined offset here:

def _datetime_to_rfc3339(value: datetime) -> str:
    """Converts the datetime to an RFC3339 string"""
    iso = value.isoformat()
    if not value.utcoffset():
        # rfc3339 needs a Z if there is no timezone offset
        iso += 'Z'
    return iso

New behavior:
The Z is tagged on only if the utcoffset() is None.

When the TZ is UTC the dt string is:
2022-06-01T01:00:00+00:00

The previous test setup for timezone aware datetimes obfuscated this bug, so I changed the tests as well.

PR Checklist:

• This PR is as small and focused as possible
• If this PR includes proposed changes for inclusion in the changelog, the title of this PR summarizes those changes and is ready for inclusion in the Changelog.
• I have updated docstrings for function changes and docs in the 'docs' folder for user interface / behavior changes
• This PR does not break any examples or I have updated them

(Optional) @mentions for Notifications:

[pl-gideon]

Thank you for fixing this issue!