update CI workflows: update/fix codecheck; remove Ubuntu 18.04 (#1167)

1. update/fix codecheck

Run workflow with pylint 2.15.7

Add a  bunch of fiddly and mostly cosmetic changes to
make pylint (and make codecheck) happy.

Also try to run pyflakes3 vs. pyflakes

2. remove Ubuntu 18.04

Ubuntu 18.04 has been removed from github actions, so
we remove it from our workflow as well

examples/cluster.py

@@ -82,14 +82,13 @@ import sys
 import re
 from itertools import groupby
 from operator import attrgetter
-from distutils.version import StrictVersion
 
 from mininet.node import Node, Host, OVSSwitch, Controller
 from mininet.link import Link, Intf
 from mininet.net import Mininet
 from mininet.topo import LinearTopo
 from mininet.topolib import TreeTopo
-from mininet.util import quietRun, errRun, decode
+from mininet.util import quietRun, errRun, decode, StrictVersion
 from mininet.examples.clustercli import CLI
 from mininet.log import setLogLevel, debug, info, error
 from mininet.clean import addCleanupCallback
@@ -314,7 +313,7 @@ class RemoteOVSSwitch( RemoteMixin, OVSSwitch ):
         kwargs.update( batch=True )
         super( RemoteOVSSwitch, self ).__init__( *args, **kwargs )
 
-    def isOldOVS( self ):
+    def isOldOVS( self ):  # pylint: disable=arguments-differ
         "Is remote switch using an old OVS version?"
         cls = type( self )
         if self.server not in cls.OVSVersions:
@@ -376,8 +375,9 @@ class RemoteLink( Link ):
             Link.stop( self )
         self.tunnel = None
 
-    def makeIntfPair( self, intfname1, intfname2, addr1=None, addr2=None,
-                      node1=None, node2=None, deleteIntfs=True   ):
+    def makeIntfPair( self,  # pylint: disable=arguments-renamed
+                      intfname1, intfname2, addr1=None, addr2=None,
+                      node1=None, node2=None, deleteIntfs=True ):
         """Create pair of interfaces
             intfname1: name of interface 1
             intfname2: name of interface 2
@@ -781,7 +781,9 @@ class MininetCluster( Mininet ):
         "Popen() for server connections"
         assert self  # please pylint
         old = signal( SIGINT, SIG_IGN )
+        # pylint: disable=consider-using-with
         conn = Popen( cmd, stdin=PIPE, stdout=PIPE, close_fds=True )
+        # pylint: enable=consider-using-with
         signal( SIGINT, old )
         return conn