stubgen (stubgenc): Support for __new__ in C types

Feature

Implement support for __new__ in mypy.stubgenc.generate_c_type_stub

Pitch

Currently, there is a # TODO: We should support __new__. in mypy.stubgenc.generate_c_type_stub.
AFAIK, C extensions autogenerate the docstring Create and return a new object. See help(type) for accurate signature. for the __new__ / __init__ attributes of C type objects and I am not aware of a way to customize this.
Therefore, my proposal would be to use the docstring of the C type itself and scan for TypeName(...) (if TypeName is the name of the C type) and pass this to infer_sig_from_docstring.
Not sure if TypeName(...) should be required to include self / cls or if such an argument should be made optional (automatically add self to the signature of __init__ if it is missing in TypeName(...)).

I would offer to implement this, if we could specify the desired behavior.

Start in mypy.stubgenc.generate_c_type_stub and read how it currently handles C type attributes and calls infer_sig_from_docstring. Resolve the intended parsing and self/cls behavior for TypeName(...) before implementing it. Done means generated C type stubs include the agreed new and init signatures derived from the type docstring.