requirements: Upgrade pyflakes to 2.0.0.

We fix a few errors that only the new version finds.

tools/test-js-with-node

@@ -141,7 +141,7 @@ try:
 except OSError:
     print('Bad command: %s' % (command,))
     raise
-except subprocess.CalledProcessError as e:
+except subprocess.CalledProcessError:
     print('\n** Tests failed, PLEASE FIX! **\n')
     sys.exit(1)