[DEPENDENCIES] Switch to PyMySQL

Our previous MySQL adapter doesn't play nicely with Gevent, and would block the entire process on queries. This became super apparent when trying to serve the static docs from the same process that was running expensive API requests.

PyMySQL is an all-Python database connector that works great with greenlets and Gevent's monkey patching. It has hundreds of stars on GitHub[1], and a vibrant community.

Technically it's a bit slower[2] than the pure C implementation, but I don't imagine that will have a significant effect for us. Effectively not blocking is way more important. Plus, it's pure Python in case we want to move to PyPy someday.

This commit also fixes a related bug where we were relying on the database adapter to coerce the uuid objects to strings.

[1] https://github.com/PyMySQL/PyMySQL

[2] https://github.com/Tin/sqlalchemy-gevent-mysql-drivers-comparison