Python2 artifacts in Python3 syntax

[gate-tec]

After the commit running 2-to-3 the base syntax of the scripts is now only supporting python3.x, however, there are some python2 exclusive expressions remaining.

For example the reference to string.letters in clean.py.

I haven't checked all files so there might be more instances of python2 expressions remaining.

For of now the current state of pdb_clean does not work on both python2 and python3.