tox4: Drop Python 3.6 support

[ymyzk]

Description

Drop support of Python 3.6: https://www.python.org/dev/peps/pep-0494/

[codecov-commenter]

Codecov Report

Merging #108 (aabfa00) into tox4 (3462583) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             tox4     #108   +/-   ##
=======================================
  Coverage   77.68%   77.68%           
=======================================
  Files           2        2           
  Lines         121      121           
  Branches       32       32           
=======================================
  Hits           94       94           
  Misses         14       14           
  Partials       13       13           

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