Add docs requirements

[sfdye]

RTD log:

Running Sphinx v1.8.5

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/python-future/envs/stable/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/python-future/envs/stable/lib/python3.7/site-packages/sphinx/util/pycompat.py", line 150, in execfile_
    exec_(code, _globals)
  File "/home/docs/checkouts/readthedocs.org/user_builds/python-future/checkouts/stable/docs/conf.py", line 17, in <module>
    import sphinx_bootstrap_theme
ModuleNotFoundError: No module named 'sphinx_bootstrap_theme'

Add doc requirements according to:
https://docs.readthedocs.io/en/stable/guides/specifying-dependencies.html