Add tests of README generation

[jwodder]

Be sure to also:

• Add py3 to the envlist in tox.ini
• Change flake src to flake src test in tox.ini
• Add {[testenv]deps} to the deps field in the typing testenv in tox.ini
• Change mypy src to mypy src test in tox.ini
• Uncomment the commented-out python-versions and toxenv in .github/workflows/test.yml