Add explicit Optional to types

[jonathangreen]

This PR adds Optional[] to type hints for parameters that can take None. This is useful when running mypy on code consuming Dunami as a library. Otherwise you get type errors from mypy when passing None as an argument to these functions.

error: Argument "metadata" to "serialize" of "Version" has incompatible type "Optional[bool]"; expected "bool"

I also updated the .pre-commit-config.yaml to run mypy with the --no-implicit-optional flag to catch parameters that should be marked as Optional in the future.

Thank you for the very helpful library! 🙏🏻

[mtkennerly]

Thanks for this! I didn't realize that this style was now deprecated. For anyone else who stops by, here's the relevant note from PEP 484:

A past version of this PEP allowed type checkers to assume an optional type when the default value is None [...] This is no longer the recommended behavior. Type checkers should move towards requiring the optional type to be made explicit.