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/Modules/clinic/_tracemalloc.c.h b/Modules/clinic/_tracemalloc.c.h
index 2ad3970..aa736ec 100644
--- a/Modules/clinic/_tracemalloc.c.h
+++ b/Modules/clinic/_tracemalloc.c.h
@@ -84,7 +84,7 @@
 "trace to nframe.");
 
 #define _TRACEMALLOC_START_METHODDEF    \
-    {"start", (PyCFunction)_tracemalloc_start, METH_FASTCALL, _tracemalloc_start__doc__},
+    {"start", (PyCFunction)(void(*)(void))_tracemalloc_start, METH_FASTCALL, _tracemalloc_start__doc__},
 
 static PyObject *
 _tracemalloc_start_impl(PyObject *module, int nframe);
@@ -185,4 +185,4 @@
 {
     return _tracemalloc_get_traced_memory_impl(module);
 }
-/*[clinic end generated code: output=d98afded69c89d52 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=d4a2dd3eaba9f72d input=a9049054013a1b77]*/