docs: update requirements for build on rtd

[dhellmann]

• Add importlib_metadata dependency for older python versions, which
  seems to be what is used on RTD.
• Split the docs job out of the other style jobs and pin the version
  of Python to force the use of the separate library in CI builds of
  docs.
• Update the mergify rules accordingly.

See https://readthedocs.org/projects/virtualenvwrapper/builds/19455841/ for an example build failure.

Running Sphinx v1.8.6

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/virtualenvwrapper/envs/latest/lib/python3.7/site-packages/sphinx/config.py", line 368, in eval_config_file
    execfile_(filename, namespace)
  File "/home/docs/checkouts/readthedocs.org/user_builds/virtualenvwrapper/envs/latest/lib/python3.7/site-packages/sphinx/util/pycompat.py", line 150, in execfile_
    exec_(code, _globals)
  File "/home/docs/checkouts/readthedocs.org/user_builds/virtualenvwrapper/checkouts/latest/docs/source/conf.py", line 16, in <module>
    import importlib.metadata
ModuleNotFoundError: No module named 'importlib.metadata'