Enable mypy and ensure we pass with strict mode

[ssbarnea]

A progressive approach would be highly desirable here, as we also have to merge a lot of new code into the repository.

• enable mypy via pre-commit with minimal mode
• activate "arg-type"
• index
• var-annotated
• union-attr
• return-value
• attr-defined
• override
• assignment
• misc
• return
• disallow_untyped_calls = true
• disallow_untyped_defs = true
• disallow_any_generics = true
• disallow_any_unimported = True
• warn_redundant_casts = True
• warn_return_any = True
• warn_unused_configs = True
• strict = true (keep this last)