Some PEP-8 'cleanup' and a little bit of variable renaming particularly in FrameMeta.__new__ to follow more conventional naming
Slight clarification of this 'hack' I think (and avoids creating one
unnecessary list).

I was thinking of adding this recipe [1] to Astropy's utils and using it
to compute the MRO, which I think is a bit nicer.  But some performance
testing showed it not to be significantly faster in most cases, so I
stuck with the temp class creation which is not too horrible after all.

[1] http://code.activestate.com/recipes/577748-calculate-the-mro-of-a-class/
Per https://github.com/astropy/astropy/pull/2957/files#r17755626 these are really two separate comments [skip-ci]
Merge pull request #2957 from embray/coordinates-pep8

Coordinates PEP-8