Use TypeVars for return types of RedisModel and its subtype's methods

[marianhlavac]

Classes RedisModel, HashModel and JsonModel had wrong return type definitions on some of their methods.
This effectively prevented static type checking and typehinting on all objects returned by these methods.

This PR adds correct type hints using TypeVars.

Before
After

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This superseeds PR #472

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 🎉

Comparison is base (70f6401) 78.94% compared to head (a8710a5) 78.96%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #476      +/-   ##
==========================================
+ Coverage   78.94%   78.96%   +0.01%     
==========================================
  Files          14       14              
  Lines        1178     1179       +1     
==========================================
+ Hits          930      931       +1     
  Misses        248      248              

Flag	Coverage Δ
unit	78.96% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aredis_om/model/model.py	87.10% <100.00%> (+0.01%)	⬆️

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[marianhlavac]

I've rebased the PR on latest main from upstream. The checks should pass now.

[marianhlavac]

Is anything else blocking this PR?

[frozenBranch]

I think this is a necessary change, is there a reason it was not merged?

[marianhlavac]

Well very obviously this project is beyond being unmaintained, which is really shame.