fix: replace deprecated utcfromtimestamp

[andriawang6]

addresses #1781

Failing tests due to not stripping tzinfo

[gkevinzheng]

Hi @andriawang6,

I don't think you can simply replace utcfromtimestamp with fromtimestamp(tz=datetime.timezone.utc) since utcfromtimestamp is not offset aware, whereas fromtimestamp is. This causes issues with comparing timestamps. We've resolved a similar issue in the past (see this discussion) by adding a .replace(tzinfo=None) after creating the timestamp object with fromtimestamp(tz=datetime.timezone.utc). You can add it to google/auth/_helpers.py like we did the function definition for utcnow

Also see this PR: #1413

[chalmerlowe]

Minor fixes. Checking on a few things in the PR.