Backport source role for linking to files in the cpython repo.

Backport source role for linking to files in the cpython repo.

Georg added this role in our 3.2 doc tools and gave the greenlight for a
backport on python-dev.

This code is a simplified version of the 3.2 code; the version of Sphinx
used with Python 2.7 doesn’t have the function used to parse markup like
:role:`text to be displayed <text to be processed>` (I was persuaded it
was a standard reST construct, but it is actually a Sphinx innovation
that has to be supported explicitly in role code —I’ll be damned).  It
is thus not possible to write for example :source:`the NEWS file
<Misc/NEWS>`, but :source:`Misc/NEWS` will work.