[PyROOT] Add stubs (*.pyi files) with type hints for ROOT classes

A very useful feature would be adding *.pyi files to the ROOT installation with type hints, which supports for example autocompletion in IDEs.

An example *.pyi file for the TH1 class would be

class TH1:

    @overload

    def __init__(self) -> None: ...

    @overload

    def __init__(self, name: str, title: str, nbinsx: int, xlow: float, xup: float) -> None: ...

Here the related PEP 484 describing type hints/annotations: https://www.python.org/dev/peps/pep-0484/