Add typings of keys and values of __members__ dict

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 / pybind11-stubgen