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/_warnings.c.h b/Python/clinic/_warnings.c.h
index 68fad23..d1e50de 100644
--- a/Python/clinic/_warnings.c.h
+++ b/Python/clinic/_warnings.c.h
@@ -9,7 +9,7 @@
 "Issue a warning, or maybe ignore it or raise an exception.");
 
 #define WARNINGS_WARN_METHODDEF    \
-    {"warn", (PyCFunction)warnings_warn, METH_FASTCALL|METH_KEYWORDS, warnings_warn__doc__},
+    {"warn", (PyCFunction)(void(*)(void))warnings_warn, METH_FASTCALL|METH_KEYWORDS, warnings_warn__doc__},
 
 static PyObject *
 warnings_warn_impl(PyObject *module, PyObject *message, PyObject *category,
@@ -35,4 +35,4 @@
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=86369ece63001d78 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=a4fbe6e2d1cc2091 input=a9049054013a1b77]*/