Drop python 3.6 support, add python 3.9 support

[rly]

Motivation

Python 3.6 end of life is 23 Dec 2021. The latest versions of h5py, numpy, pandas, scipy do not provide wheels for python 3.6. It seems that generally, if a package added 3.9 support, they dropped 3.6 support. This makes it difficult to test against.

This PR will merge into the enh/h5py3 branch to drop python 3.6 support and add python 3.9 support.

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

Merging #620 (3188154) into enh/h5py3 (b55bd7c) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           enh/h5py3     #620   +/-   ##
==========================================
  Coverage      86.09%   86.09%           
==========================================
  Files             41       41           
  Lines           8268     8268           
  Branches        1780     1780           
==========================================
  Hits            7118     7118           
  Misses           814      814           
  Partials         336      336           

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[rly]

The pynwb-dev-python39 test fails because Python 3.9 is not yet supported by PyNWB (but would be after this PR and a corresponding one in PyNWB).

[rly]

I could not find an early release of ruamel.yaml in the 0.15.* range that worked for Python 3.7 and all OS's, for use as a minimum version requirement for ruamel.yaml, so I bumped up the minimum requirement to 0.16. If there is a need, we can explore what versions in the 0.15.* range would work.