Link to plaintext for "show source" links (GH-137131)
(cherry picked from commit 9cbf46d992)
Co-authored-by: ryan-duve <ryan-duve@users.noreply.github.com>
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"
* bpo-28929: Link the documentation to its source file on GitHub
Change the documentation's `Show Source` link on the left menu
to GitHub source file.
* change version to master
