Deprecate Py3.6 and Py3.7

[maxfischer2781]

• Remove Py3.6 and Py3.7 from automation workflows.
• Remove Py3.6 and Py3.7 from package metadata.
• Remove typing_extensions dependency.
• Various general cleanup as part of reviewing all modules:
  • remove use of slot_get helper (closes slot_get helper works incorrectly with metaclasses #78)
  • improved type inference for reduce function arguments
  • improved typing (and ignores) to better cover Pylance in addition to MyPy

Closes #95.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (77a16f6) 99.09% compared to head (1ffdace) 98.88%.

❗ Current head 1ffdace differs from pull request most recent head 6c9cd8f. Consider uploading reports for the commit 6c9cd8f to get more accurate results

Files	Patch %	Lines
asyncstdlib/_lrucache.py	71.42%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
- Coverage   99.09%   98.88%   -0.21%     
==========================================
  Files          20       20              
  Lines        2097     2069      -28     
  Branches      556      511      -45     
==========================================
- Hits         2078     2046      -32     
- Misses         11       15       +4     
  Partials        8        8              

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