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

Include/internal/pycore_modsupport.h

@@ -13,6 +13,24 @@ extern int _PyArg_NoKwnames(const char *funcname, PyObject *kwnames);
 #define _PyArg_NoKwnames(funcname, kwnames) \
     ((kwnames) == NULL || _PyArg_NoKwnames((funcname), (kwnames)))
 
+// Export for '_bz2' shared extension
+PyAPI_FUNC(int) _PyArg_NoPositional(const char *funcname, PyObject *args);
+#define _PyArg_NoPositional(funcname, args) \
+    ((args) == NULL || _PyArg_NoPositional((funcname), (args)))
+
+// Export for '_asyncio' shared extension
+PyAPI_FUNC(int) _PyArg_NoKeywords(const char *funcname, PyObject *kwargs);
+#define _PyArg_NoKeywords(funcname, kwargs) \
+    ((kwargs) == NULL || _PyArg_NoKeywords((funcname), (kwargs)))
+
+// Export for 'zlib' shared extension
+PyAPI_FUNC(int) _PyArg_CheckPositional(const char *, Py_ssize_t,
+                                       Py_ssize_t, Py_ssize_t);
+#define _Py_ANY_VARARGS(n) ((n) == PY_SSIZE_T_MAX)
+#define _PyArg_CheckPositional(funcname, nargs, min, max) \
+    ((!_Py_ANY_VARARGS(max) && (min) <= (nargs) && (nargs) <= (max)) \
+     || _PyArg_CheckPositional((funcname), (nargs), (min), (max)))
+
 extern PyObject ** _Py_VaBuildStack(
     PyObject **small_stack,
     Py_ssize_t small_stack_len,
@@ -22,6 +40,80 @@ extern PyObject ** _Py_VaBuildStack(
 
 extern PyObject* _PyModule_CreateInitialized(PyModuleDef*, int apiver);
 
+// Export for '_curses' shared extension
+PyAPI_FUNC(int) _PyArg_ParseStack(
+    PyObject *const *args,
+    Py_ssize_t nargs,
+    const char *format,
+    ...);
+
+extern int _PyArg_UnpackStack(
+    PyObject *const *args,
+    Py_ssize_t nargs,
+    const char *name,
+    Py_ssize_t min,
+    Py_ssize_t max,
+    ...);
+
+// Export for '_heapq' shared extension
+PyAPI_FUNC(void) _PyArg_BadArgument(
+    const char *fname,
+    const char *displayname,
+    const char *expected,
+    PyObject *arg);
+
+// --- _PyArg_Parser API ---------------------------------------------------
+
+typedef struct _PyArg_Parser {
+    int initialized;
+    const char *format;
+    const char * const *keywords;
+    const char *fname;
+    const char *custom_msg;
+    int pos;            /* number of positional-only arguments */
+    int min;            /* minimal number of arguments */
+    int max;            /* maximal number of positional arguments */
+    PyObject *kwtuple;  /* tuple of keyword parameter names */
+    struct _PyArg_Parser *next;
+} _PyArg_Parser;
+
+// Export for '_testclinic' shared extension
+PyAPI_FUNC(int) _PyArg_ParseTupleAndKeywordsFast(PyObject *, PyObject *,
+                                                 struct _PyArg_Parser *, ...);
+
+// Export for '_dbm' shared extension
+PyAPI_FUNC(int) _PyArg_ParseStackAndKeywords(
+    PyObject *const *args,
+    Py_ssize_t nargs,
+    PyObject *kwnames,
+    struct _PyArg_Parser *,
+    ...);
+
+// Export for 'math' shared extension
+PyAPI_FUNC(PyObject * const *) _PyArg_UnpackKeywords(
+    PyObject *const *args,
+    Py_ssize_t nargs,
+    PyObject *kwargs,
+    PyObject *kwnames,
+    struct _PyArg_Parser *parser,
+    int minpos,
+    int maxpos,
+    int minkw,
+    PyObject **buf);
+#define _PyArg_UnpackKeywords(args, nargs, kwargs, kwnames, parser, minpos, maxpos, minkw, buf) \
+    (((minkw) == 0 && (kwargs) == NULL && (kwnames) == NULL && \
+      (minpos) <= (nargs) && (nargs) <= (maxpos) && (args) != NULL) ? (args) : \
+     _PyArg_UnpackKeywords((args), (nargs), (kwargs), (kwnames), (parser), \
+                           (minpos), (maxpos), (minkw), (buf)))
+
+// Export for '_testclinic' shared extension
+PyAPI_FUNC(PyObject * const *) _PyArg_UnpackKeywordsWithVararg(
+        PyObject *const *args, Py_ssize_t nargs,
+        PyObject *kwargs, PyObject *kwnames,
+        struct _PyArg_Parser *parser,
+        int minpos, int maxpos, int minkw,
+        int vararg, PyObject **buf);
+
 #ifdef __cplusplus
 }
 #endif