Enable more of mypy's diagnostics

[SamWilsn]

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.

[Praniti1594]

I'd like to work on it. Can I be assigned to it?

[github-actions]

This issue has had no activity for 60 days and has been marked as stale.