Move to using the sqlalchemy mypy plugin

[PeterJCLaw]

This provides better type checking against the sqlalchemy code and allows us to remove our stubs and the type: ignores which were in various places where the stubs didn't match reality.

Included here for now are some related refactors which this highlighted, though they should probably be hoisted to separate PRs.

[coveralls]

Coverage increased (+0.0003%) to 99.346% when pulling 8a5f8f9f79e1a11b44570c6f498579c3a6a5a9a0 on use-sqlalchemy-mypy-plugin into 7048fea08042c21dccfe37f316fe6c1f3e9c84f4 on more-types.