Drop support for running libcst using a python 3.6 interpreter

[lpetre]

Summary

Python 3.6 security support ended in December 2021. Some major libraries (including some transitive dependencies) have stopped supporting 3.6.

[zsol]

Readme needs some changing too

[codecov-commenter]

Codecov Report

Merging #663 (2a95270) into main (f863feb) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #663      +/-   ##
==========================================
+ Coverage   94.79%   94.81%   +0.01%     
==========================================
  Files         249      249              
  Lines       25456    25448       -8     
==========================================
- Hits        24132    24128       -4     
+ Misses       1324     1320       -4     

Impacted Files	Coverage Δ
setup.py	88.88% <ø> (ø)
libcst/_type_enforce.py	93.54% <100.00%> (+4.97%)	⬆️

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