bpo-33012: Fix invalid function cast warnings with gcc 8. (GH-6749)

Fix invalid function cast warnings with gcc 8
for method conventions different from METH_NOARGS, METH_O and
METH_VARARGS excluding Argument Clinic generated code.
diff --git a/Modules/_elementtree.c b/Modules/_elementtree.c
index 2f1c4c0..62374d8 100644
--- a/Modules/_elementtree.c
+++ b/Modules/_elementtree.c
@@ -3968,7 +3968,7 @@
 /* python module interface */
 
 static PyMethodDef _functions[] = {
-    {"SubElement", (PyCFunction) subelement, METH_VARARGS | METH_KEYWORDS},
+    {"SubElement", (PyCFunction)(void(*)(void)) subelement, METH_VARARGS | METH_KEYWORDS},
     {NULL, NULL}
 };