Changes in pex_config and pipe_base

Not being very familiar with this part of the codebase it's not that obvious how this works, but assuming it does it looks OK to me, other than the one comment about the line length of the comment being too long. And normally I wouldn't be so picky, but I believe that's going to stop builds working soon, so best not to add more places where that rule is broken.

I take it you've run this through Jenkins etc? Assuming so, it's good to merge once you fix that comment length

Fixed the long line (although why comments don't follow the same rules as all other code I don't know – I sort-of understand the argument for doc strings).