Enable more of mypy's diagnostics

The full list is here: https://mypy.readthedocs.io/en/stable/error_code_list2.html

From a quick read, we probably want to enable at least:

- `disallow-any-generics`
- `disallow-untyped-defs`
- `warn-redundant-casts`
- `strict-equality`
- `disallow-untyped-calls`
- `warn-return-any`
- `disallow-any-unimported`
- `warn-unreachable`
- `ignore-without-code`
- `unused-ignore`
- `explicit-override`
- `mutable-override`

I think some of these are already on, but it doesn't hurt to check.