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.

Python/context.c

@@ -648,7 +648,7 @@ static PyMethodDef PyContext_methods[] = {
     _CONTEXTVARS_CONTEXT_KEYS_METHODDEF
     _CONTEXTVARS_CONTEXT_VALUES_METHODDEF
     _CONTEXTVARS_CONTEXT_COPY_METHODDEF
-    {"run", (PyCFunction)context_run, METH_FASTCALL | METH_KEYWORDS, NULL},
+    {"run", (PyCFunction)(void(*)(void))context_run, METH_FASTCALL | METH_KEYWORDS, NULL},
     {NULL, NULL}
 };