Add set_trace snippet

[jgarte]

This PR removes the previous snippet bound to pu that I added. and introduces a new snippet bound to tr which is short for trace as in set_trace(). I think this is a better use of tempel and allows one to modularize the snippet over pdb, ipdb, and pudb.

For example, with pdb you can do the following:

import pdb; pdb.set_trace()

with ipdb:

import ipdb; ipdb.set_trace()

with pudb

import pudb; pudb.set_trace()