Fix #429 readthedocs build

[mkeilman]

Updates pykern to conform to the new readthedocs process:

• adds .readthdocs.yml with required settings (and see below)
• adds docs/requirements.txt and excepting it from docs/.gitignore
• creates a ReadTheDocs command class that encapsulates the methods previously invoked directly by setup() and writes docs/conf.py

These are the commands that readthedocs runs to build documents:

git clone --depth 1 https://github.com/radiasoft/pykern.git .
git fetch origin --force --prune --prune-tags --depth 50 refs/heads/<branch>:refs/remotes/origin/<branch>
git checkout --force origin/<branch>
git clean -d -f -f
cat .readthedocs.yml
asdf global python 3.9.18
python -mvirtualenv $READTHEDOCS_VIRTUALENV_PATH
python -m pip install --upgrade --no-cache-dir pip setuptools
python -m pip install --upgrade --no-cache-dir sphinx readthedocs-sphinx-ext
python -m pip install --exists-action=w --no-cache-dir -r docs/requirements.txt
<commands in .readthedocs.yml/build/jobs/pre_build>
cat docs/conf.py
python -m sphinx -T -E -b html -d _build/doctrees -D language=en . $READTHEDOCS_OUTPUT/html

In our case the only thing in docs/requirements.txt is ".", meaning install pykern itself including its requirements as listed in setup.py._requires(), and the only pre-build command is python setup.py readthedocs to call the new command class.

Another note: we still need sphinx-apidoc to generate the .rst files. It's actually available on readthedocs but we'll do it in the command class so we have control over the packages.

YAN: python setup.py tox fails even in master. It's not related to this issue so I did not address it.

YAN: We may be able to ditch _readthedocs_fixup() but I'm not sure...