Use identity operators for comparison to None singleton.

PEP 8 specifies:

* Comparisons to singletons like None should always be done with `is` or
  `is not`, never the equality operators.

auth-mastodon.py

@@ -138,16 +138,16 @@ def get_password(user, host):
     cursor.execute(db_query_getpass, {"user": user.lower(), "host": host})
     data = cursor.fetchone()
     cursor.close()
-    return data[0] if data != None else None
+    return data[0] if data is not None else None
 
 
 def isuser(user, host):
-    return get_password(user, host) != None
+    return get_password(user, host) is not None
 
 
 def auth(user, host, password):
     db_password = get_password(user, host)
-    if db_password == None:
+    if db_password is None:
         logging.debug("Wrong username: %s@%s" % (user, host))
         return False
     else: