pep8: Fix E711, comparisons to None should use 'is' or 'is not'

Lengthy discussion of why this is a good thing (I didn't know) at SO:

http://stackoverflow.com/questions/2209755/python-operation-vs-is-not

mininet/log.py

@@ -117,7 +117,7 @@ class MininetLogger( Logger, object ):
            Convenience function to support lowercase names.
            levelName: level name from LEVELS"""
         level = LOGLEVELDEFAULT
-        if levelname != None:
+        if levelname is not None:
             if levelname not in LEVELS:
                 raise Exception( 'unknown levelname seen in setLogLevel' )
             else:

mininet/net.py

@@ -421,7 +421,7 @@ class Mininet( object ):
             return (1, 0)
         r = r'(\d+) packets transmitted, (\d+) received'
         m = re.search( r, pingOutput )
-        if m == None:
+        if m is None:
             error( '*** Error: could not parse ping output: %s\n' %
                      pingOutput )
             return (1, 0)

mininet/util.py

@@ -46,7 +46,7 @@ def oldQuietRun( *cmd ):
                 break
             out += data
         popen.poll()
-        if popen.returncode != None:
+        if popen.returncode is not None:
             break
     return out