[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.
`install` is just a bit more compact than `mkdir` and `chmod`. That's why I'm using it. If you think `mkdir` and `chmod` should be used instead, I'll revert this change.