gh-110964: Remove private _PyArg functions (#110966)

Move the following private functions and structures to
pycore_modsupport.h internal C API:

* _PyArg_BadArgument()
* _PyArg_CheckPositional()
* _PyArg_NoKeywords()
* _PyArg_NoPositional()
* _PyArg_ParseStack()
* _PyArg_ParseStackAndKeywords()
* _PyArg_Parser structure
* _PyArg_UnpackKeywords()
* _PyArg_UnpackKeywordsWithVararg()
* _PyArg_UnpackStack()
* _Py_ANY_VARARGS()

Changes:

* Python/getargs.h now includes pycore_modsupport.h to export
  functions.
* clinic.py now adds pycore_modsupport.h when one of these functions
  is used.
* Add pycore_modsupport.h includes when a C extension uses one of
  these functions.
* Define Py_BUILD_CORE_MODULE in C extensions which now include
  directly or indirectly (via code generated by Argument Clinic)
  pycore_modsupport.h:

  * _csv
  * _curses_panel
  * _dbm
  * _gdbm
  * _multiprocessing.posixshmem
  * _sqlite.row
  * _statistics
  * grp
  * resource
  * syslog

* _testcapi: bad_get() no longer uses METH_FASTCALL calling
  convention but METH_VARARGS. Replace _PyArg_UnpackStack() with
  PyArg_ParseTuple().
* _testcapi: add PYTESTCAPI_NEED_INTERNAL_API macro which is defined
  by _testcapi sub-modules which need the internal C API
  (pycore_modsupport.h): exceptions.c, float.c, vectorcall.c,
  watchers.c.
* Remove Include/cpython/modsupport.h header file.
  Include/modsupport.h no longer includes the removed header file.
* Fix mypy clinic.py

Modules/_multiprocessing/clinic/multiprocessing.c.h

@@ -2,6 +2,8 @@
 preserve
 [clinic start generated code]*/
 
+#include "pycore_modsupport.h"    // _PyArg_CheckPositional()
+
 #if defined(MS_WINDOWS)
 
 PyDoc_STRVAR(_multiprocessing_closesocket__doc__,
@@ -166,4 +168,4 @@ exit:
 #ifndef _MULTIPROCESSING_SEND_METHODDEF
     #define _MULTIPROCESSING_SEND_METHODDEF
 #endif /* !defined(_MULTIPROCESSING_SEND_METHODDEF) */
-/*[clinic end generated code: output=8b91c020d4353cc5 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=48504f7a2d37958c input=a9049054013a1b77]*/

Modules/_multiprocessing/clinic/posixshmem.c.h

@@ -6,6 +6,7 @@ preserve
 #  include "pycore_gc.h"          // PyGC_Head
 #  include "pycore_runtime.h"     // _Py_ID()
 #endif
+#include "pycore_modsupport.h"    // _PyArg_UnpackKeywords()
 
 #if defined(HAVE_SHM_OPEN)
 
@@ -165,4 +166,4 @@ exit:
 #ifndef _POSIXSHMEM_SHM_UNLINK_METHODDEF
     #define _POSIXSHMEM_SHM_UNLINK_METHODDEF
 #endif /* !defined(_POSIXSHMEM_SHM_UNLINK_METHODDEF) */
-/*[clinic end generated code: output=2f356903a281d857 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=b525631f5915d7f3 input=a9049054013a1b77]*/

Modules/_multiprocessing/clinic/semaphore.c.h

@@ -6,6 +6,7 @@ preserve
 #  include "pycore_gc.h"          // PyGC_Head
 #  include "pycore_runtime.h"     // _Py_ID()
 #endif
+#include "pycore_modsupport.h"    // _PyArg_UnpackKeywords()
 
 #if defined(HAVE_MP_SEMAPHORE) && defined(MS_WINDOWS)
 
@@ -541,4 +542,4 @@ exit:
 #ifndef _MULTIPROCESSING_SEMLOCK___EXIT___METHODDEF
     #define _MULTIPROCESSING_SEMLOCK___EXIT___METHODDEF
 #endif /* !defined(_MULTIPROCESSING_SEMLOCK___EXIT___METHODDEF) */
-/*[clinic end generated code: output=e8ea65f8cba8e173 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=d57992037e6770b6 input=a9049054013a1b77]*/

Modules/_multiprocessing/posixshmem.c

@@ -2,6 +2,11 @@
 posixshmem - A Python extension that provides shm_open() and shm_unlink()
 */
 
+// clinic/posixshmem.c.h uses internal pycore_modsupport.h API
+#ifndef Py_BUILD_CORE_BUILTIN
+#  define Py_BUILD_CORE_MODULE 1
+#endif
+
 #include <Python.h>
 
 // for shm_open() and shm_unlink()