DOC: Making doc/source/conf.py pass PEP-8, and added to lint
- [X] closes https://github.com/pandas-dev/pandas/issues/19838
- [ ] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

For some reason, travis CI is not starting for this PR. Made an edit to re-trigger it, but still nothing .. I tested this locally, so merging (so I can rebase https://github.com/pandas-dev/pandas/pull/19840)