[TODO] Get rid of the disabled MyPy error codes

[webknjaz]

1. Go through the disable_error_code entries in .mypy.ini
2. Remove them one by one, and fix whatever they report in the actual Python modules
3. Multiple small PRs are welcome and are preferred over a single gigantic one
4. It should be easier to start with the test modules
5. If there's just one violation in the module, move the disabled error code into that module inline, if it's not fixable
6. If some rules are hard to fix/understand, switch to the easier ones first