Remove the unnecessary self return converter in `winreg.c`

[aisk]

Bug report

Bug description:

There is an Argument Clinic converter (marked as “HACK” in the comment) in winreg.c:

cpython/PC/winreg.c

Lines 268 to 277 in a275385

	# HACK: this only works for PyHKEYObjects, nothing else.
	# Should this be generalized and enshrined in clinic.py,
	# destroy this converter with prejudice.
	class self_return_converter(CReturnConverter):
	type = 'PyHKEYObject *'
	def render(self, function, data):
	self.declare(data)
	data.return_conversion.append(
	'return_value = (PyObject *)_return_value;\n')

It is only used in the HKEY.__enter__ method. In that method, the type of the return value PyObject * is cast to a PyHKEYObject *:

cpython/PC/winreg.c

Lines 335 to 344 in a275385

	/*[clinic input]
	winreg.HKEYType.__enter__ -> self
	[clinic start generated code]*/
	static PyHKEYObject *
	winreg_HKEYType___enter___impl(PyHKEYObject *self)
	/*[clinic end generated code: output=52c34986dab28990 input=c40fab1f0690a8e2]*/
	{
	return (PyHKEYObject*)Py_XNewRef(self);
	}

What the self_return_converter does is simply cast the PyHKEYObject * back to PyObject *, which is unnecessary.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Linked PRs

• gh-142579: Remove the self converter hack in winreg.c #142580