fix(type hints): remove notimplemented as type hints as not valid

[binste]

NotImplemented is an instance and the actual type is NotImplementedType. The mypy errors which this fixes look like this:

...
ibis/expr/types/numeric.py:711: error: Variable "builtins.NotImplemented" is not valid as a type  [valid-type]
ibis/expr/types/numeric.py:711: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
...

As mentioned in #6844, I can create a few more of these smaller PRs or I can bundle more changes into a single PR. Whatever works best for you.

Edit: NotImplementedType was introduced in Python 3.10 so tests fail for 3.9. I'll look into it.

[binste]

In Python < 3.10, NotImplemented cannot be typed at all as mypy does not accept type(NotImplemented) as a type hint. Apparently it is in general unclear if and how to type hint NotImplemented, see python/mypy#363.

Based on some testing, this SO thread, and looking into other libraries such as pandas-stubs, my understanding is that newer versions of mypy work well if NotImplementedType is not provided as a return type but instead only the type of the object that is returned if the operation was successful.

[cpcloud]

LGTM! Thank you :)