Enable sip pyi generated

[echoix]

Fixes #NNNN

[timrid]

I like this proposal more than the one from here as it seems much more complete. I tested this briefly and noticed the following points:

1. Missing Constants
The most errors in my codebase are because of missing constants. Many constants are now moved to a subclass (which i generally like) but they are not a global variables any more. I think this is needed for backward compatiblity reasons.

2. Some methods not found
Some methods that are defined in python code and are dynamically added have no type hints. Eg. wx.Window.Bind.

3. Some functions are not correctly typed:
The type hints for functions that are defined in python code (eg wx.CallAfter) are wrong.

4. Setter of @property seems strange
Currently the setter of @property have mutliple arguments like this:

class Window(WindowBase):
...
    @property
    def Size(self) -> Size: ...
    @Size.setter
    def Size(self, x: int, y: int, width: int, height: int, sizeFlags: int = ...) -> None: ...
...

But they have to have only one argument like this:

class Window(WindowBase):
...
    @property
    def Size(self) -> Size: ...
    @Size.setter
    def Size(self, val: Size) -> None: ...
...

4. Missing documentation
The docstrings in the pyi files are missing.

[echoix]

My goal here wasn't to replace the ones already there by the PI generator, but to add the info from the sip-generated ones into the PI generated ones (that's why in the PI generator, I make the classes be a subclass of the class from the one in the module extension). This allows to keep the same docstrings generated.

Thus, the methods defined in Python in the real py files (eg core.py), it is expected that they don't appear from sip-generated files.

[echoix]

But your observation about properties is interesting