Type hinting __aenter__ with Self

[jamesbraza]

Adds typing-extensions>=4 for Python before 3.11 for the Self type, too.

Closes #352

[codecov]

Codecov Report

❌ Patch coverage is 55.55556% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.89%. Comparing base (8d28ac7) to head (e4e0f4f).
⚠️ Report is 39 commits behind head on main.

Files with missing lines	Patch %	Lines
aiohttp_client_cache/response.py	60.00%	1 Missing and 1 partial ⚠️
aiohttp_client_cache/session.py	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #367      +/-   ##
==========================================
- Coverage   96.20%   95.89%   -0.32%     
==========================================
  Files          10       10              
  Lines        1081     1096      +15     
  Branches      188      113      -75     
==========================================
+ Hits         1040     1051      +11     
- Misses         30       32       +2     
- Partials       11       13       +2     

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[JWCook]

Thanks for contributing!