Travis test on Windows, and against CPython3.9, 3.10, pypy3

[tartley]

Python3.9, 3.10, and pypy3.

• Add these to tox config. See https://docs.travis-ci.com/user/languages/python/
• Make sure tests all pass
• Add these to supported python versions at top of README
• Check this para, which mentions making builds parallelizable while using tox. Ours don't seem to be. https://docs.travis-ci.com/user/languages/python/#using-tox-as-the-build-script
• Review which OS is being used for our tests on Travis. If it isn't Windows, that seems like a mistake. See https://docs.travis-ci.com/user/languages/python/#running-python-tests-on-multiple-operating-systems

[jdufresne]

Some of this is handled in #289 and #290