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/Objects/stringlib/clinic/transmogrify.h.h b/Objects/stringlib/clinic/transmogrify.h.h
index 6d26622..fb63060 100644
--- a/Objects/stringlib/clinic/transmogrify.h.h
+++ b/Objects/stringlib/clinic/transmogrify.h.h
@@ -11,7 +11,7 @@
 "If tabsize is not given, a tab size of 8 characters is assumed.");
 
 #define STRINGLIB_EXPANDTABS_METHODDEF    \
-    {"expandtabs", (PyCFunction)stringlib_expandtabs, METH_FASTCALL|METH_KEYWORDS, stringlib_expandtabs__doc__},
+    {"expandtabs", (PyCFunction)(void(*)(void))stringlib_expandtabs, METH_FASTCALL|METH_KEYWORDS, stringlib_expandtabs__doc__},
 
 static PyObject *
 stringlib_expandtabs_impl(PyObject *self, int tabsize);
@@ -43,7 +43,7 @@
 "Padding is done using the specified fill character.");
 
 #define STRINGLIB_LJUST_METHODDEF    \
-    {"ljust", (PyCFunction)stringlib_ljust, METH_FASTCALL, stringlib_ljust__doc__},
+    {"ljust", (PyCFunction)(void(*)(void))stringlib_ljust, METH_FASTCALL, stringlib_ljust__doc__},
 
 static PyObject *
 stringlib_ljust_impl(PyObject *self, Py_ssize_t width, char fillchar);
@@ -74,7 +74,7 @@
 "Padding is done using the specified fill character.");
 
 #define STRINGLIB_RJUST_METHODDEF    \
-    {"rjust", (PyCFunction)stringlib_rjust, METH_FASTCALL, stringlib_rjust__doc__},
+    {"rjust", (PyCFunction)(void(*)(void))stringlib_rjust, METH_FASTCALL, stringlib_rjust__doc__},
 
 static PyObject *
 stringlib_rjust_impl(PyObject *self, Py_ssize_t width, char fillchar);
@@ -105,7 +105,7 @@
 "Padding is done using the specified fill character.");
 
 #define STRINGLIB_CENTER_METHODDEF    \
-    {"center", (PyCFunction)stringlib_center, METH_FASTCALL, stringlib_center__doc__},
+    {"center", (PyCFunction)(void(*)(void))stringlib_center, METH_FASTCALL, stringlib_center__doc__},
 
 static PyObject *
 stringlib_center_impl(PyObject *self, Py_ssize_t width, char fillchar);
@@ -155,4 +155,4 @@
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=336620159a1fc70d input=a9049054013a1b77]*/
+/*[clinic end generated code: output=d09ba158d470566e input=a9049054013a1b77]*/