Fix issue #3912 : invalid docstring generation of std::array <-> list caster

[Skylion007]

Description

• Fixes #3912, by removing size information from list, while this information is helpful, it generates invalid type signatures for mypy and other type checkers.

Suggested changelog entry:

* Fixes doc generation for std::array-list caster. Previously, signatures included the size of the list in a non-standard, non-spec compliant way.

[wjakob]

This PR achieves MyPy compliance in exchange for potentially very confusing situations where a pybind11-bound function does not accept an input array, and the error message provides zero context why that is (input has the wrong size)

[wjakob]

These signatures in pybind11 precede the time where MyPy was even a thing, so "non-standard non-spec compliant" seems a bit strong of a qualifier.

[Skylion007]

@Wjacob, in >=3.9 we can use the Annotated typing extension to specify the length of the list. Hmm, not sure if there is a standard compliant way to specify the type for anything besides Tuples though.

[cielavenir]

@Skylion007 @rwgk can we close this one?

[rwgk]

Closing after #4679 was merged.