ctypes._FuncPointer doesn't exist at runtime and _ctypes.CFuncPtr doesn't exist in typestub

[junkmd]

Quoting from #9251 (comment);

• Our stub has a _FuncPointer class that doesn't exist at runtime. But in fact, the code returns a subclass of _ctypes.CFuncPtr, so why don't we use that class in the stubs?

In addition, Python's ctypes runtime namespace is defined by the following lines;

• from _ctypes import CFuncPtr as _CFuncPtr importing part
  https://github.com/python/cpython/blob/872cbc613245db7a1fc5e6656ed0135d2e115f50/Lib/ctypes/__init__.py#L10

  • Subclasses of _ctypes.CFuncPtr is created by the following lines

    • Definition of CFUNCTYPE, factory function that creates dynamically CFunctionType class
      https://github.com/python/cpython/blob/872cbc613245db7a1fc5e6656ed0135d2e115f50/Lib/ctypes/__init__.py#L73-L107

    • Definition of PYFUNCTYPE, factory function that creates dynamically CFunctionType class
      https://github.com/python/cpython/blob/872cbc613245db7a1fc5e6656ed0135d2e115f50/Lib/ctypes/__init__.py#L509-L514

    • Definition of WINFUNCTYPE, factory function that creates dynamically WinFunctionType class
      https://github.com/python/cpython/blob/872cbc613245db7a1fc5e6656ed0135d2e115f50/Lib/ctypes/__init__.py#L114-L134

    • Definition of CDLL, its' __getitem__ and __getattr__ return _FuncPtr class
      https://github.com/python/cpython/blob/872cbc613245db7a1fc5e6656ed0135d2e115f50/Lib/ctypes/__init__.py#L322-L400

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I think the following way can be improved with less changes and less impact.

1. move the definition of _FuncPointer to _ctypes.pyi.
2. change the class name to CFuncPtr.
3. add from _ctypes import CFuncPtr as _CFuncPtr line to ctypes/__init__.pyi.
4. add class _FuncPointer(_CFuncPtr): ... line.

The ctypes._FuncPointer will be conceptualized as "a dynamically generated subclass of _ctypes.CFuncPtr".

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this issue is related to #8968, #8571 (comment) and #8633 (comment)

[junkmd]

#10140 has been merged and the definition movings are completed.