Add typings of keys and values of __members__ dict

[jasminlapalme]

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

For example, with this pybind11 code of library named lib :

py::enum_<Number>(m, "Number")
  .value("Zero", Number::Zero)
  .value("One", Number::One)
  .value("Tow", Number::Two);

instead of :

class Number():
  ....
  __members__: dict # value = {'Zero': <Number.Zero: 0>, 'One': < Number.One: 1>, 'Two': <Number.Two: 2>}

to have :

class Number():
  ....
  __members__: dict[str, lib.Number] # value = {'Zero': <Number.Zero: 0>, 'One': < Number.One: 1>, 'Two': <Number.Two: 2>}

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

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

[sizmailov]

Thanks for the suggestion!

Implemented in #112

Example from tests:

pybind11-stubgen/tests/stubs/demo/_bindings/enum.pyi

Lines 39 to 41 in a1cd5fa

	__members__: typing.ClassVar[
	typing.Dict[str, ConsoleForegroundColor]
	] # value = {'Green': <ConsoleForegroundColor.Green: 32>, 'Yellow': <ConsoleForegroundColor.Yellow: 33>, 'Blue': <ConsoleForegroundColor.Blue: 34>, 'Magenta': <ConsoleForegroundColor.Magenta: 35>}