bpo-33012: Fix invalid function cast warnings with gcc 8 in Argument Clinic. (GH-6748)

Fix invalid function cast warnings with gcc 8
for method conventions different from METH_NOARGS, METH_O and
METH_VARARGS in Argument Clinic generated code.
diff --git a/Python/clinic/import.c.h b/Python/clinic/import.c.h
index 27b83ec..1eae8f2 100644
--- a/Python/clinic/import.c.h
+++ b/Python/clinic/import.c.h
@@ -75,7 +75,7 @@
 "    File path to use.");
 
 #define _IMP__FIX_CO_FILENAME_METHODDEF    \
-    {"_fix_co_filename", (PyCFunction)_imp__fix_co_filename, METH_FASTCALL, _imp__fix_co_filename__doc__},
+    {"_fix_co_filename", (PyCFunction)(void(*)(void))_imp__fix_co_filename, METH_FASTCALL, _imp__fix_co_filename__doc__},
 
 static PyObject *
 _imp__fix_co_filename_impl(PyObject *module, PyCodeObject *code,
@@ -269,7 +269,7 @@
 "Create an extension module.");
 
 #define _IMP_CREATE_DYNAMIC_METHODDEF    \
-    {"create_dynamic", (PyCFunction)_imp_create_dynamic, METH_FASTCALL, _imp_create_dynamic__doc__},
+    {"create_dynamic", (PyCFunction)(void(*)(void))_imp_create_dynamic, METH_FASTCALL, _imp_create_dynamic__doc__},
 
 static PyObject *
 _imp_create_dynamic_impl(PyObject *module, PyObject *spec, PyObject *file);
@@ -360,7 +360,7 @@
 "\n");
 
 #define _IMP_SOURCE_HASH_METHODDEF    \
-    {"source_hash", (PyCFunction)_imp_source_hash, METH_FASTCALL|METH_KEYWORDS, _imp_source_hash__doc__},
+    {"source_hash", (PyCFunction)(void(*)(void))_imp_source_hash, METH_FASTCALL|METH_KEYWORDS, _imp_source_hash__doc__},
 
 static PyObject *
 _imp_source_hash_impl(PyObject *module, long key, Py_buffer *source);
@@ -396,4 +396,4 @@
 #ifndef _IMP_EXEC_DYNAMIC_METHODDEF
     #define _IMP_EXEC_DYNAMIC_METHODDEF
 #endif /* !defined(_IMP_EXEC_DYNAMIC_METHODDEF) */
-/*[clinic end generated code: output=f0660cd1de6b3a73 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=ad747b76e105fff2 input=a9049054013a1b77]*/