Use rtd config file with htmldir builder

[hugovk]

Updates and closes #493.
Replaces and closes #784.

This adds the newline as requested in #493 and makes some other updates:

• The htmldir builder option was renamed to dirhtml to use the same name as sphinx.

  • https://docs.readthedocs.io/en/stable/config-file/v2.html?#sphinx-builder

• The python.version option is now deprecated and replaced by build.tools.python.

  • https://docs.readthedocs.io/en/stable/config-file/v2.html?#python-version

• Let's also bump from Python 3.7 to latest 3.10.

  • https://docs.readthedocs.io/en/stable/config-file/v2.html?#build-tools-python

[hugovk]

Error
Problem in your project's configuration. Invalid "build.tools.python": .readthedocs.yml: Invalid configuration option: build.tools.python. Make sure the key name is correct.

https://readthedocs.org/projects/cpython-devguide/builds/15708889/

Will check!

[hugovk]

build.os was missing and is required.

• https://docs.readthedocs.io/en/stable/config-file/v2.html?#build-os

Now passing and building. Also rebased to use the updated theme:

• https://cpython-devguide--789.org.readthedocs.build/

And update Sphinx to be able to use Python 3.10:

• Sphinx 3.x broken on Python 3.10 sphinx-doc/sphinx#9816

And let's update Sphinx and Furo whilst we're at it.

[zware]

With @JulienPalard having already approved #493 and the build looking fine to me, merging.