RFC: Drop support for Python 3.6

[duckontheweb]

Related Issue(s):

• Closes Deprecating support for older Python 3 versions #474

Description:

Drops support for Python 3.6 (both in terms of python_version check on install and in CI tests), and adds a note to the README that we follow the NEP-29 guidelines.

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[duckontheweb]

cc: @matthewhanson @gadomski @lossyrob @TomAugspurger

[lossyrob]

I believe @schwehr is still stuck on 3.6. I'd rather not remove 3.6 if that will be really disruptive to his usage of PySTAC as I believe he is a big user and has been a great contributor to the project. If that's not the case, I'm +1.

[schwehr]

Am in the process of going py 3.6 to 3.7. Go for it. +1 for dropping 3.6. Thanks for asking me!

[duckontheweb]

Just a note that I also fixed this as part of this PR.

[codecov-commenter]

Codecov Report

Merging #500 (c6bf951) into main (c0f0b6d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #500   +/-   ##
=======================================
  Coverage   95.43%   95.43%           
=======================================
  Files          71       71           
  Lines        9978     9978           
=======================================
  Hits         9523     9523           
  Misses        455      455           

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