Fix utcnow deprecation warning

[MatthewScholefield]

I'm seeing the following warning:

lib/python3.12/site-packages/fastapi_users/jwt.py:25: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).
    expire = datetime.utcnow() + timedelta(seconds=lifetime_seconds)

To fix it I changed to the timezone aware version. Since it gets serialized directly below I don't think there should be any impact from this change.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (2ffb700) to head (783bfe5).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1369   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           35        35           
  Lines          976       976           
=========================================
  Hits           976       976           

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[frankie567]

Good catch, thank you @MatthewScholefield 🙏

@all-contributors add @MatthewScholefield for code

[allcontributors]

@frankie567

I've put up a pull request to add @MatthewScholefield! 🎉