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Since version 1.5.0, Sphinx is adding the source file extension to the source files it includes in the output folder.
For example, when your source file extension is `rst` (specified in the config variable `source_suffix` as `[.rst]`, your index file appears in the output's source folder as `index.rst.txt`.
If you use a Sphinx version lower than 1.5.0, it appears as `index.txt`.
Here's the link to the PR that introduces the breaking change: sphinx-doc/sphinx2454.
The solution adjust the file name/suffix based on the Sphinx version:
* If you use Sphinx >=1.5, the file name will be filename + source_file_ending + ".txt".
* If you use Sphinx < 1.5, the file name will be filename + ".txt".
The changes also add aconfig option that allows forcing a fallback to 2).
This will help folks who use this extension for a long time and whose templates evolved accordingly.