Support only Python 3.9 and higher

[kurtmckee]

This PR introduces the following changes:

• A python_requires option in setup.py restricts installs to Python 3.9+
• PyPI trove classifiers, and a note in the docs index, advertise Python 3.9+ support
• Tox and CI now test against Python 3.9+ across all platforms.

Closes #177

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This PR begins the work to standardize testing across tools and environments.

There are at least five different ways to run the test suite, each with different commands:

• The commands in the Makefile (pytest or unittest+coverage)
• The commands in tox.ini (unittest+coverage)
• CI for Linux and macOS (pytest+coverage)
• CI for Windows (unittest)
• Manual invocation (?)

To begin standardizing things, the Linux and macOS CI setup commands are now selected using in-YAML conditions (if:) rather than using bash conditionals. This helps prepare for the Windows CI job to integrate directly in with Linux and macOS.

Future standardization will eliminate pytest/unittest/coverage invocation differences.

[coveralls]

coverage: 28.845%. remained the same
when pulling df3fbfe on kurtmckee:support-py39
into 9436613 on LudovicRousseau:master.

[LudovicRousseau]

Thanks