Tentative support for Python 3.9

[thibaudcolas]

This updates the library’s test suite and package metadata to check for Python 3.9 support. No code changes needed – but there were a few deprecation warnings in html5lib, and in the example code. This should make it easier to support Python 3.10 in late 2021.

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• Stay on point and keep it small so it can be easily reviewed. For example, try to apply any general refactoring separately outside of the PR.
• [ ] Consider adding unit tests, especially for bug fixes. If you don't, tell us why. N/A
• All new and existing tests pass, with 100% test coverage (make test-coverage)
• Linting passes (make lint)
• Consider updating documentation. If you don't, tell us why.
• List the environments / platforms in which you tested your changes. Python 3.9.0b5