Fix SyntaxWarning on Python 3.8

supervisor/medusa/filesys.py:218
  /home/travis/build/Supervisor/supervisor/supervisor/medusa/filesys.py:218: SyntaxWarning: "is not" with a literal. Did you mean "!="?
    if self.persona is not (None, None):

supervisor/medusa/filesys.py:225
  /home/travis/build/Supervisor/supervisor/supervisor/medusa/filesys.py:225: SyntaxWarning: "is not" with a literal. Did you mean "!="?
    if self.persona is not (None, None):

Author: mnaberez

supervisor/medusa/filesys.py

@@ -215,14 +215,14 @@ def __init__ (self, root, wd='/', persona=(None, None)):
             self.persona = persona
 
         def become_persona (self):
-            if self.persona is not (None, None):
+            if self.persona != (None, None):
                 uid, gid = self.persona
                 # the order of these is important!
                 os.setegid (gid)
                 os.seteuid (uid)
 
         def become_nobody (self):
-            if self.persona is not (None, None):
+            if self.persona != (None, None):
                 os.seteuid (self.PROCESS_UID)
                 os.setegid (self.PROCESS_GID)