PYTHON-4690 Add repr for FixedOffset eg FixedOffset(datetime.timedelta(seconds=3600), '+60'))

[ShaneHarvey]

PYTHON-4690

Motivation:
When working with multiple timezones, this change makes it easier to see which ones are being used.

Before:

>>> from bson.tz_util import FixedOffset, utc
>>> utc
<bson.tz_util.FixedOffset object at 0x104ec2840>
>>> FixedOffset(60, 'tzName')
<bson.tz_util.FixedOffset object at 0x104b0cc20>

After:

>>> from bson.tz_util import FixedOffset, utc
>>> utc
FixedOffset(datetime.timedelta(0), 'UTC')
>>> FixedOffset(60, 'tzName')
FixedOffset(datetime.timedelta(seconds=3600), 'tzName')

Related to PYTHON-4663.

[blink1073]

LGTM!