[DONOTMERGE] Improve Python 3 compat, clean up code

Further improve compatibility with Python 3 by making the `urlparse` import
conditional.

Make some sed expressions use "|" as a separator instead of "/", thus allowing
all occurrences of "\/" to be removed.

Update some comments and shuffle around code to make it more compact and
readable. Add some FIXMEs.
:+1: I don't know this can be done, I still need to think a little when reading, but is better to avoid escaping all `/ `