Add Python 3.11, Remove Python 3.7

[AaronDJohnson]

As is typical, the day before the next version of Python is released, I'm recommending that we support Python 3.11 and remove Python 3.7 which is deprecated.

This PR adds support for Python 3.11, and removes support for Python 3.7.
Additionally, it updates the Python versions wherever they are mentioned (e.g., in the docs where Python 2.6 was mentioned).

[codecov]

Codecov Report

Merging #362 (294dd75) into master (c49646c) will decrease coverage by 0.07%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #362      +/-   ##
==========================================
- Coverage   88.37%   88.31%   -0.07%     
==========================================
  Files          13       13              
  Lines        3012     3012              
==========================================
- Hits         2662     2660       -2     
- Misses        350      352       +2     

see 1 file with indirect coverage changes

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