[3.5] Fix broken Show Source links on documentation pages (GH-3113)

[Mariatta]

The Show Source was broken because of a change made in sphinx 1.5.1
In Sphinx 1.4.9, the sourcename was "index.txt".
In Sphinx 1.5.1+, it is now "index.rst.txt".
(cherry picked from commit b9ff498)

[Mariatta]

@larryhastings Show Source link on docs.python.org on the 3.5 branch is currently broken.
For example, go to: https://docs.python.org/3.5/library/index.html
and click the Show Source link on the left.

I think there are two ways to fix it, but not sure what you think:

• accept this PR
• rebuild the python docs using Sphinx 1.4.9

Ideas?

[Mariatta]

@larryhastings or @ned-deily can look into this?

[ned-deily]

Looked good to me. Thanks!

[Mariatta]

Thanks 😄