Use dirhtml builder instead of default html

[hugovk]

My previous PR #88 added a Read the Docs config file, which inadvertently switched from dirhtml builder (the default without a config file) to html (the default with a config file: https://docs.readthedocs.io/en/stable/config-file/v2.html#sphinx-builder). Sorry about that!

For example:

• before, dirhtml: python -m sphinx -T -E -b dirhtml -d _build/doctrees -D language=en . $READTHEDOCS_OUTPUT/html
  https://readthedocs.org/projects/pypaio/builds/20072068/
• after, html: python -m sphinx -T -E -b html -d _build/doctrees -D language=en . $READTHEDOCS_OUTPUT/html https://readthedocs.org/projects/pypaio/builds/20093063/

And here's someone (python/peps#3199) noticing a previously working https://www.pypa.io/en/latest/specifications/ is now 404 and the working one is https://www.pypa.io/en/latest/specifications.html

Let's explicitly switch back to dirhtml to ensure other existing links aren't broken. (And we can revert python/peps#3199.)