Make datetime.tzinfo non-optional for TZAware

[antonagestam]

This PR makes it so that accessing .tzinfo on a datetime.datetime object that has been narrowed to TZAware has the type datetime.tzinfo instead of datetime.tzinfo | None. This is safe because this check is already part of the predicate, ie by definition of isinstance(_, TZAware).

It also addresses #84 for TZAware and TZNaive, by checking that accessing an attribute from datetime.datetime does not raise a static type checking error.

[codecov-commenter]

Codecov Report

Merging #233 (e160e7d) into main (0cadc67) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #233   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           24        24           
  Lines          820       821    +1     
  Branches       172       172           
=========================================
+ Hits           820       821    +1     

Impacted Files	Coverage Δ
src/phantom/datetime.py	100.00% <100.00%> (ø)

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