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/multipoll.py

@@ -18,11 +18,12 @@ from mininet.util import decode
 
 def monitorFiles( outfiles, seconds, timeoutms ):
     "Monitor set of files and return [(host, line)...]"
-    devnull = open( '/dev/null', 'w' )
+    devnull = open( '/dev/null', 'w' )  # pylint: disable=consider-using-with
     tails, fdToFile, fdToHost = {}, {}, {}
     for h, outfile in outfiles.items():
-        tail = Popen( [ 'tail', '-f', outfile ],
-                      stdout=PIPE, stderr=devnull )
+        tail = Popen(  # pylint: disable=consider-using-with
+            [ 'tail', '-f', outfile ],
+            stdout=PIPE, stderr=devnull )
         fd = tail.stdout.fileno()
         tails[ h ] = tail
         fdToFile[ fd ] = tail.stdout