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]

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 ⚠️
githubkit/versions/v2022_11_28/rest/__init__.py	85.71%	1 Missing ⚠️
githubkit/versions/v2022_11_28/rest/repos.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@           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	Coverage Δ
unittests	75.60% <89.28%> (+<0.01%)	⬆️

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