diagnose: Fix exception with Python 3.8+ versions

The "psutil._exceptions" has been removed at some point, leading to a
CLI exception whenever OpenSIPS performs a down-scale operation, leading
to a PIDfile being reported as not found.

opensipscli/modules/diagnose.py

@@ -738,7 +738,7 @@ def diagnose_transport_load(self, transport, pgroups, load, net_wait):
                 if have_psutil:
                     try:
                         tot_cpu += proc['cpumon'].cpu_percent(interval=None)
-                    except psutil._exceptions.NoSuchProcess:
+                    except psutil.NoSuchProcess:
                         """opensips may be restarted in the meantime!"""
 
             avg_cpu = round(tot_cpu / len(procs))