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

bin/mn

@@ -189,8 +189,9 @@ class MininetRunner( object ):
             customs = {}
             if os.path.isfile( fileName ):
                 # pylint: disable=exec-used
-                exec( compile( open( fileName ).read(), fileName, 'exec' ),
-                      customs, customs )
+                with open( fileName ) as f:
+                    exec( compile( f.read(), fileName, 'exec' ),
+                          customs, customs )
                 for name, val in customs.items():
                     self.setCustom( name, val )
             else: