Two minor issues:

This script is written in python two which is depreciated, and should be rewritten in python three, but I have so far been successful in not learning python. :-) to simplify debugging I have clarified that it needs a python two interpreter.

This script downloads an old version of mesa (an update may be appropriate),  unfortunately the release candidate previously specified is no longer available, so an update to the next released version was made.

This makes it easier to handle common exceptions. I was motivated
because the Mesa URL is currently broken and wasn't handled well.

Implementation mostly copied from here:
  http://stackoverflow.com/questions/22676/how-do-i-download-a-file-over-http-using-python

Old UTF-8 resulted in this error from GNU Emacs:
  Warning (mule): Invalid coding system `UTF-8' is specified
  for the current buffer/file by the :coding tag.
  It is highly recommended to fix it before writing to a file.

We're not doing a sufficient job of keeping this updated so list
it as an external requirement instead.