Drop support for Python 3.6

[reoono]

What does this PR do?

Updated documentation, setup.py, and CI to drop support for 3.6.

Fixes #665

[codecov]

Codecov Report

Merging #666 (09ee319) into main (76906a8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #666   +/-   ##
=======================================
  Coverage   91.18%   91.18%           
=======================================
  Files          17       17           
  Lines        1486     1486           
=======================================
  Hits         1355     1355           
  Misses        131      131           

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[MSeal]

Thanks!