DOC,BLD: Sphinx >= 2.0 support

[westurner]

• https://github.com/tell-k/sphinxjp.themes.basicstrap/pull/25 fixes the app.info logging
  • [x] patch locally pending this getting merged
• Now I'm getting sphinx.errors.ThemeError: no theme named 'basicstrap' found (missing theme.conf?)
  • pip install requirements.txt does pip install -e git+ssh://github.com/tell-k/sphinxjp.themes.basicstrap#egg=basicstrap
  • I've tried:
    • pip install -e .
    • pip install .

For now, the workaround is to:

pip install 'sphinx<2.0'