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

---

This superseeds PR https://github.com/redis/redis-om-python/pull/472

[codecov-commenter]

Codecov Report

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

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

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Additional details and impacted files

```diff @@ 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%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=redis#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://app.codecov.io/gh/redis/redis-om-python/pull/476?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=redis) | Coverage Δ | | |---|---|---| | [aredis\_om/model/model.py](https://app.codecov.io/gh/redis/redis-om-python/pull/476?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=redis#diff-YXJlZGlzX29tL21vZGVsL21vZGVsLnB5) | `87.10% <100.00%> (+0.01%)` | :arrow_up: |

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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.