Support newer docutils versions

[kurtmckee]

This PR introduces the following changes, based on feedback in #310:

• Drop support for Python 3.8

  (Newer docutils releases drop support for Python 3.8 and change how some test files render)

• Require docutils 0.21.2 or higher

• Update three HTML test files to match how docutils 0.21.2 now renders the accompanying input files

• Test all supported CPython and PyPy versions in tox and in CI

• Remove <s> from the list of allowed HTML tags, now that newer docutils versions don't render that tag anymore

Fixes #310

[!NOTE]

If this merges, #311 can be closed.