fix: replace deprecated utcfromtimestamp

[andriawang6]

addresses #1781

• Moves away from using utcfromtimestamp() in the datetime library due to deprecation.
• Adds a new helper function to manage internal handling of timezone aware versus timezone naive timestamps.

DeprecationWarning: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.fromtimestamp(timestamp, datetime.UTC).

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

[Linchin]

I think we need to add unit test for utcfromtimestamp() in tests/test__helpers.py.

[chalmerlowe]

Removed the Do-Not-Merge label after discussion with @parthea. Further work on this PR appears to have resolved the issue in question as identified by Kevin: #1799 (comment)

[chalmerlowe]

I think we need to add unit test for utcfromtimestamp() in tests/test__helpers.py.
[From this comment](#1799 (review)

@Linchin
Added unit tests.

[chalmerlowe]

LGTM

[parthea]

Following up on offline discussion, tests pass regardless of the changes in tests_async/oauth2/test__client_async.py. Revert this file as it appears to be noise

[parthea]

use _helpers.utcfromtimestamp . Applies throughout

[parthea]

Switching to draft while review comments are being addressed