Pass code check with pylint=2.4.4 (#1012)

examples/hwintf.py

@@ -8,6 +8,8 @@ hardware interface) to a network after the network is created.
 import re
 import sys
 
+from sys import exit  # pylint: disable=redefined-builtin
+
 from mininet.cli import CLI
 from mininet.log import setLogLevel, info, error
 from mininet.net import Mininet
@@ -15,6 +17,7 @@ from mininet.link import Intf
 from mininet.topolib import TreeTopo
 from mininet.util import quietRun
 
+
 def checkIntf( intf ):
     "Make sure intf exists and is not configured."
     config = quietRun( 'ifconfig %s 2>/dev/null' % intf, shell=True )
@@ -27,6 +30,7 @@ def checkIntf( intf ):
                'and is probably in use!\n' )
         exit( 1 )
 
+
 if __name__ == '__main__':
     setLogLevel( 'info' )