555: Add support for Python 3.9

[lindsay-stevens]

Closes #555

Why is this the best possible solution? Were any other approaches considered?

A GitHub Actions yaml file was in place already with a release job for creating a build with Py3.8 and linux (ubuntu) and uploading it to PyPI. This PR updates the yaml to add jobs for:

• lint, with Py3.8 on linux, by running black, isort, flake8, and pycodestyle
• test, with Py3.7, 3.8, and 3.9 on linux, macos, and windows, by running nosetests
  • At the end of the test job the sdist and wheel are uploaded as Artifacts (example here).

The above more or less covers what the AppVeyor and CircleCI configurations are doing. So if this all looks good I could add removal of those CI dotfiles to this PR to close out #568 as well.

Some minor changes were required for Py3.9 compatibility. These are noted in e9c77ab and 3b21ce9, and involved removing a method call and a kwarg which were deprecated since Py3.2 and Py3.1 respectively.

What are the regression risks?

Unlikely, but maybe the test CI environments are slightly different. But the tests pass all the same.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No, it would only affect developers.

Before submitting this PR, please make sure you have:

• [x] included test cases for core behavior and edge cases in tests
• [x] run nosetests and verified all tests pass
• [x] run black pyxform tests to format code
• [x] verified that any code or assets from external sources are properly credited in comments

[codecov-commenter]

Codecov Report

Merging #580 (3b21ce9) into master (bb2497d) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #580   +/-   ##
=======================================
  Coverage   83.59%   83.59%           
=======================================
  Files          25       25           
  Lines        3761     3761           
  Branches      930      930           
=======================================
  Hits         3144     3144           
  Misses        474      474           
  Partials      143      143           

Impacted Files	Coverage Δ
pyxform/utils.py	78.08% <100.00%> (ø)

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[lindsay-stevens]

./.circleci/config.yml and ./appveyor.yml have been deleted so these checks failed. ./.coveragerc also deleted.