Fix compatibility with older Python versions

[sworisbreathing]

• add a fallback to typing_extensions when importing Self
• use pre-3.10 Union types

Hopefully this should fix #17 (at least for 3.9+)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (7cbf31a) to head (3f8e826).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #18   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines          677       680    +3     
  Branches        56        56           
=========================================
+ Hits           677       680    +3     

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[2bndy5]

Thanks for looking into this. I should also add older python versions to the CI workflow for verification.

[2bndy5]

I should also add older python versions to the CI workflow for verification.

I have a branch ready for this, but need this merged first.