Hey Guix,

Here's a Mesa update for the just released 18.3.1 version. Not sure
whether or not I'm too late for the staging branch. If I am this can
wait until the next staging branch.

Two things to note:
* While Mesa 18.3.x still builds with LLVM 6 the upcoming 19.0.x series
  doesn't anymore. For that and performance reasons I decided to add a
  llvm@7.0.0 package for Mesa. I don't know enough about the LLVM stack
  to upgrade it entirely, so this is kind of a workaround.
* Since Mesa 18.3 works with Python 3 I've replaced the Python 2
  dependencies with Python 3 ones.