Clean up requirements, etc.

[jGaboardi]

This PR:

• corrects supports supported Python versions info in setup.py
• updates and alphabetizes all .ci/ environments
• updates and alphabetizes environment.yml
• updates requirements{_tests,docs}.txt
• removes requirements_dev.txt since it is unused and not in MANIFEST.in

[codecov-commenter]

Codecov Report

Merging #177 (6761349) into master (97effe3) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 6761349 differs from pull request most recent head 44cb65a. Consider uploading reports for the commit 44cb65a to get more accurate results

@@           Coverage Diff           @@
##           master     #177   +/-   ##
=======================================
  Coverage   63.49%   63.49%           
=======================================
  Files         118      118           
  Lines        4421     4421           
=======================================
  Hits         2807     2807           
  Misses       1614     1614           

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[knaaptime]

looking soild. Is this ready james?