fix _ctypes.CFuncPtr constructor args

[junkmd]

Quoting from #9251 (comment)

• All of the arguments are positional-only and should be marked as such in the stub.

-> 35b1273

• We're missing an overload for the case where no arguments are passed to the constructor.

-> e36b517

• The overload with vtbl_index (not vtlb_index as the stub calls it) should exist only on Windows.

-> 9d7beb0, e86ea2e

I fixed them.

---

• Another PR will be submitted for what is dealt with in The constructor of _FuncPointer can accept None to paramflags and Structure to iid in runtime. #8968.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

comtypes (https://github.com/enthought/comtypes)
+ comtypes/_memberspec.py:357: note:     def __init__(self) -> _FuncPointer
- comtypes/_memberspec.py:357: note:     def __init__(self, address: int) -> _FuncPointer
+ comtypes/_memberspec.py:357: note:     def __init__(self, int, /) -> _FuncPointer
- comtypes/_memberspec.py:357: note:     def __init__(self, callable: Callable[..., Any]) -> _FuncPointer
+ comtypes/_memberspec.py:357: note:     def __init__(self, Callable[..., Any], /) -> _FuncPointer
- comtypes/_memberspec.py:357: note:     def __init__(self, func_spec: Tuple[Union[str, int], CDLL], paramflags: Tuple[Union[Tuple[int], Tuple[int, str], Tuple[int, str, Any]], ...] = ...) -> _FuncPointer
+ comtypes/_memberspec.py:357: note:     def __init__(self, Tuple[Union[str, int], CDLL], Tuple[Union[Tuple[int], Tuple[int, str], Tuple[int, str, Any]], ...] = ..., /) -> _FuncPointer
- comtypes/_memberspec.py:357: note:     def __init__(self, vtlb_index: int, name: str, paramflags: Tuple[Union[Tuple[int], Tuple[int, str], Tuple[int, str, Any]], ...] = ..., iid: _Pointer[c_int] = ...) -> _FuncPointer
+ comtypes/_memberspec.py:357: note:     def __init__(self, int, str, Tuple[Union[Tuple[int], Tuple[int, str], Tuple[int, str, Any]], ...] = ..., _Pointer[c_int] = ..., /) -> _FuncPointer
+ comtypes/_memberspec.py:358: note:     def __init__(self) -> _FuncPointer
- comtypes/_memberspec.py:358: note:     def __init__(self, address: int) -> _FuncPointer
+ comtypes/_memberspec.py:358: note:     def __init__(self, int, /) -> _FuncPointer
- comtypes/_memberspec.py:358: note:     def __init__(self, callable: Callable[..., Any]) -> _FuncPointer
+ comtypes/_memberspec.py:358: note:     def __init__(self, Callable[..., Any], /) -> _FuncPointer
- comtypes/_memberspec.py:358: note:     def __init__(self, func_spec: Tuple[Union[str, int], CDLL], paramflags: Tuple[Union[Tuple[int], Tuple[int, str], Tuple[int, str, Any]], ...] = ...) -> _FuncPointer
+ comtypes/_memberspec.py:358: note:     def __init__(self, Tuple[Union[str, int], CDLL], Tuple[Union[Tuple[int], Tuple[int, str], Tuple[int, str, Any]], ...] = ..., /) -> _FuncPointer
- comtypes/_memberspec.py:358: note:     def __init__(self, vtlb_index: int, name: str, paramflags: Tuple[Union[Tuple[int], Tuple[int, str], Tuple[int, str, Any]], ...] = ..., iid: _Pointer[c_int] = ...) -> _FuncPointer
+ comtypes/_memberspec.py:358: note:     def __init__(self, int, str, Tuple[Union[Tuple[int], Tuple[int, str], Tuple[int, str, Any]], ...] = ..., _Pointer[c_int] = ..., /) -> _FuncPointer