Drop support for end-of-life Python 3.7 and 3.8

[johnthagen]

Description

Drop support for end-of-life Python 3.7 and 3.8

Closes #544

Checklist

Please update this checklist as you complete each item:

• Tests have been developed for bug fixes or new functionality.
• The changelog has been updated, if necessary.
• Documentation has been updated, if necessary.
• GitHub Issues closed by this PR have been linked.

_{By submitting this pull request I agree that all contributions comply with this project's open source license(s).}

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.52%. Comparing base (dd22aa3) to head (43bead8).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #563   +/-   ##
=======================================
  Coverage   91.52%   91.52%           
=======================================
  Files          19       19           
  Lines         932      932           
  Branches      161      138   -23     
=======================================
  Hits          853      853           
  Misses         43       43           
  Partials       36       36           

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[Archmonger]

Approved with the assumption that there are no references to Django 3.7/3.8 in the docs.

[johnthagen]

Approved with the assumption that there are no references to Django 3.7/3.8 in the docs.

I could not find any. There are other unrelated things in the docs that need fixed (like references to Travis and Python 2 in the contributing guide) but I could not find anything about these Python versions.