Backward compatible version of the datetime.UTC attribute

[tonybaloney]

The PR #72 was not backward compatible. datetime.UTC was only introduced recently but it an alias for datetime.timezone.utc

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (dd4be60) 92.40% compared to head (067a0b9) 92.42%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #76      +/-   ##
==========================================
+ Coverage   92.40%   92.42%   +0.01%     
==========================================
  Files           5        5              
  Lines         540      541       +1     
==========================================
+ Hits          499      500       +1     
  Misses         41       41              

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