Type (almost) all the things

[PeterJCLaw]

This applies the result of running the tests under MonkeyType to the source code, resulting in a lot of the remaining type annotations being added.

It also makes mypy slightly stricter in now requiring that all ignores have error codes, that they do ignore something and fixes the cases that this found.

This does leave one typing issue unresolved as it is highlighting a potential bug -- if a label's metadata isn't a dict then there can be issues when merging in new metadata.

[coveralls]

Coverage decreased (-0.1%) to 99.345% when pulling 7048fea08042c21dccfe37f316fe6c1f3e9c84f4 on more-types into 6c69148203575112fff4246f243d8c663459fb78 on master.