Bug: fix cache and cycle ref issue

[yanyongyu]

Use weakref to avoid cycle ref. This can improve the gc collection. Instance can be collected immediately after var release now.

Also fix #79 . Cache now depends on github instance weakref.

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (8a381cb) 75.60% compared to head (f5612df) 75.60%.

Files	Patch %	Lines
githubkit/versions/rest.py	92.85%	1 Missing :warning:
githubkit/versions/v2022_11_28/rest/__init__.py	85.71%	1 Missing :warning:
githubkit/versions/v2022_11_28/rest/repos.py	85.71%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #81 +/- ## ======================================= Coverage 75.60% 75.60% ======================================= Files 541 541 Lines 59462 59482 +20 ======================================= + Hits 44957 44974 +17 - Misses 14505 14508 +3 ``` | [Flag](https://app.codecov.io/gh/yanyongyu/githubkit/pull/81/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Ju4tCode) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/yanyongyu/githubkit/pull/81/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Ju4tCode) | `75.60% <89.28%> (+<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=Ju4tCode#carryforward-flags-in-the-pull-request-comment) to find out more.

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