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

Tools/clinic/clinic.py

@@ -1344,6 +1344,8 @@ class CLanguage(Language):
                             """,
                         indent=4))
             else:
+                clinic.add_include('pycore_modsupport.h',
+                                   '_PyArg_CheckPositional()')
                 parser_code = [normalize_snippet(f"""
                     if (!_PyArg_CheckPositional("{{name}}", {nargs}, {min_pos}, {max_args})) {{{{
                         goto exit;
@@ -1401,6 +1403,8 @@ class CLanguage(Language):
                 if limited_capi:
                     fastcall = False
                 if fastcall:
+                    clinic.add_include('pycore_modsupport.h',
+                                       '_PyArg_ParseStack()')
                     parser_code = [normalize_snippet("""
                         if (!_PyArg_ParseStack(args, nargs, "{format_units}:{name}",
                             {parse_arguments})) {{
@@ -1429,6 +1433,11 @@ class CLanguage(Language):
 
             has_optional_kw = (max(pos_only, min_pos) + min_kw_only < len(converters) - int(vararg != NO_VARARG))
             if vararg == NO_VARARG:
+                # FIXME: refactor the code to not declare args_declaration
+                # if limited_capi is true
+                if not limited_capi:
+                    clinic.add_include('pycore_modsupport.h',
+                                       '_PyArg_UnpackKeywords()')
                 args_declaration = "_PyArg_UnpackKeywords", "%s, %s, %s" % (
                     min_pos,
                     max_pos,
@@ -1436,6 +1445,9 @@ class CLanguage(Language):
                 )
                 nargs = "nargs"
             else:
+                if not limited_capi:
+                    clinic.add_include('pycore_modsupport.h',
+                                       '_PyArg_UnpackKeywordsWithVararg()')
                 args_declaration = "_PyArg_UnpackKeywordsWithVararg", "%s, %s, %s, %s" % (
                     min_pos,
                     max_pos,
@@ -1574,6 +1586,8 @@ class CLanguage(Language):
                         declarations += "\nPy_ssize_t nargs = PyTuple_Size(args);"
 
                 elif fastcall:
+                    clinic.add_include('pycore_modsupport.h',
+                                       '_PyArg_ParseStackAndKeywords()')
                     parser_code = [normalize_snippet("""
                         if (!_PyArg_ParseStackAndKeywords(args, nargs, kwnames, &_parser{parse_arguments_comma}
                             {parse_arguments})) {{
@@ -1581,6 +1595,8 @@ class CLanguage(Language):
                         }}
                         """, indent=4)]
                 else:
+                    clinic.add_include('pycore_modsupport.h',
+                                       '_PyArg_ParseTupleAndKeywordsFast()')
                     parser_code = [normalize_snippet("""
                         if (!_PyArg_ParseTupleAndKeywordsFast(args, kwargs, &_parser,
                             {parse_arguments})) {{
@@ -1626,12 +1642,16 @@ class CLanguage(Language):
 
             if not parses_keywords:
                 declarations = '{base_type_ptr}'
+                clinic.add_include('pycore_modsupport.h',
+                                   '_PyArg_NoKeywords()')
                 fields.insert(0, normalize_snippet("""
                     if ({self_type_check}!_PyArg_NoKeywords("{name}", kwargs)) {{
                         goto exit;
                     }}
                     """, indent=4))
                 if not parses_positional:
+                    clinic.add_include('pycore_modsupport.h',
+                                       '_PyArg_NoPositional()')
                     fields.insert(0, normalize_snippet("""
                         if ({self_type_check}!_PyArg_NoPositional("{name}", args)) {{
                             goto exit;
@@ -3500,6 +3520,8 @@ class CConverter(metaclass=CConverterAutoRegister):
         else:
             if expected_literal:
                 expected = f'"{expected}"'
+            if clinic is not None:
+                clinic.add_include('pycore_modsupport.h', '_PyArg_BadArgument()')
             return f'_PyArg_BadArgument("{{{{name}}}}", "{displayname}", {expected}, {{argname}});'
 
     def format_code(self, fmt: str, *,