stubgen: Add typings of keys and values of dict (__members__, __entries)

It would be useful to add typings of the keys and values of the __members__ and __entries dictionary on class that represent enum in C.

For example, if we take the same example as #12717.

#include <cstddef>
#include <memory>
#include <pybind11/pybind11.h>

namespace py = pybind11;

enum class MyEnum { FOO, BAR };

PYBIND11_MODULE(native_enum_test, module) {
  pybind11::enum_<MyEnum>(module, "MyEnum", pybind11::arithmetic())
      .value("FOO", MyEnum::FOO)
      .value("BAR", MyEnum::BAR);
}

Compile via:

$ c++ -O3 -Wall -shared -std=c++17 -fPIC $(python3 -m pybind11 --includes) native_enum_test.cpp -o native_enum_test.so

Run the stub generator:

stubgen -p native_enum_test

the result is something like

class MyEnum:
    __members__: ClassVar[dict] = ...  # read-only
    BAR: ClassVar[MyEnum] = ...
    FOO: ClassVar[MyEnum] = ...
    __entries: ClassVar[dict] = ...
...

When we import code with these kinds of stubs and use mypy to type checked the code with strict option, we have this warning.

error: Missing type parameters for generic type "dict"  [type-arg]

Expected Behavior

The result of stubgen would be like this:

class MyEnum:
    __members__: ClassVar[dict[str, MyEnum]] = ...  # read-only
    BAR: ClassVar[MyEnum] = ...
    FOO: ClassVar[MyEnum] = ...
    __entries: ClassVar[dict[str, tuple[MyEnum, None]]] = ... # not sure about the type of this one...
...

Environment

• Mypy version used: 1.4.1
• Mypy command-line flags: not relevant, this is about the stub generator --show-column-numbers --no-error-summary --no-pretty --no-color-output
• Mypy configuration options from mypy.ini (and other config files): strict = true
• Python version used: 3.9

Start with the stubgen implementation and reproduce the report using the provided C++ enum example, then run stubgen -p native_enum_test. The work is done when generated __members__ and __entries annotations include appropriate type parameters and the resulting stubs no longer trigger the reported strict mypy warning.