cast: Add is_generic_type<T>

[EricCousineau-TRI]

Description

Adds explicit way to denote a is_generic_type<>, which is effectively bullet point (1) from the docs:
https://pybind11.readthedocs.io/en/stable/advanced/cast/index.html

Permits distinction from bullet points (2) and (3)

Suggested changelog entry:

Adds is_generic_type<> trait

[EricCousineau-TRI]

From @rwgk in #2950

Two questions:

1. Will this work if I add another specialization pair for is_base_of<smart_holder_type_caster_base_tag, ...>?

2. What do you think about choosing a different name, to connect to the intended purpose instead of pegging the name on type_caster_generic, e.g. is_iterable_t_compatible? (Otherwise it will get really confusing that generic is connected to smart_holder_type_caster.)

(1) Yup! Can simply add the specialization and it should work out of the box.
(2) I know that was for prior PR; I believe this trait is more agnostic. See PR overview for rationale.

[rwgk]

Approving this change although I have a some uncertainties.

• Could you please say something about the "why" in the PR description? Is the static_assert in isinstance the main motivation? What accidents does that prevent?

• The naming seems to refer back to class generic_type. Is that a correct understanding? Would it make sense to add a comment to explain? — Pegging is_generic_type on the type_caster seems like "circumstantial evidence" (as opposed to direct evidence). Is a direct is_generic_type too difficult to implement? My guess: yes, but it would be good to explain (tersely is fine).

• class generic_type is in pybind11.h

• A struct is_generic_type forward declaration is in pytypes.h

• The is_generic_type specializations are in cast.h

This makes me wonder: is the isinstance overload for detail::isinstance_generic in the right place? Could it be moved to cast.h?

I'm OK if the answer is no, the current pybind11 code organization has me wishing a lot and fixing that is obviously a bigger project. But if moving the isinstance overload is an option, that might help at least a little bit.