Sqlalchemy-pg-fts

- Filter additional punctuation related characters from websearch queries.

- Relax python version constraint for 3.6.

- Cleanup
- Fix `websearch` tokenization on Python 3.6

- Add `TSVector` as a column type.
- Add `TSQuery` as a column type.
- Add `to_tsvector` function.
- Add `to_tsquery` function.
- Add `websearch` to render tsquery text.