Use of string.strip() does not work in Python 3

In pipeline.py the string module is imported and there are a few case where the following style is used:

opt = string.strip(o)

The strip() method (along with other string methods have been deprecated and removed in Python 3). These case should just be changed to, e.g.,

opt = o.strip()

cc @duncanmmacleod

PS - I would have created an MR for this, but I've reached the limit of git.ligo.org repos that I'm allowed, so I can't fork glue!

refs lalsuite!466 (merged)