Added support for Python 3.9

[sliverc]

Fixes #841

Description of the Change

Added support for Python 3.9

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[n2ygk]

Looks like python 3.9 is still not available for Travis. Perhaps try 3.9-dev? Or just wait bit until Travis catches up? 3.9.0 was released Oct 5 so not sure how much longer it's going to take.

This exists: https://storage.googleapis.com/travis-ci-language-archives/python/binaries/ubuntu/16.04/x86_64/python-3.9-dev.tar.bz2

[codecov]

Codecov Report

Merging #846 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #846   +/-   ##
=======================================
  Coverage   97.77%   97.77%           
=======================================
  Files          58       58           
  Lines        3104     3104           
=======================================
  Hits         3035     3035           
  Misses         69       69           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f59bb0d...1d4f3e4. Read the comment docs.

[n2ygk]

@sliverc We can circle back later when Travis catches up and switch it back from 3.9-dev to 3.9.