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/syslogmodule.c b/Modules/syslogmodule.c
index a5807dc..b2ea73b 100644
--- a/Modules/syslogmodule.c
+++ b/Modules/syslogmodule.c
@@ -238,7 +238,7 @@
 /* List of functions defined in the module */
 
 static PyMethodDef syslog_methods[] = {
-    {"openlog",         (PyCFunction) syslog_openlog,           METH_VARARGS | METH_KEYWORDS},
+    {"openlog",         (PyCFunction)(void(*)(void)) syslog_openlog,           METH_VARARGS | METH_KEYWORDS},
     {"closelog",        syslog_closelog,        METH_NOARGS},
     {"syslog",          syslog_syslog,          METH_VARARGS},
     {"setlogmask",      syslog_setlogmask,      METH_VARARGS},