[2.2.x] Added requirements.txt to files ignored by Sphinx builds.

Backport of 0cf2d48ba83543b16bdf390d941eb98e8d34f3bd from stable/3.2.x.

docs/conf.py

@@ -118,7 +118,7 @@ today_fmt = '%B %d, %Y'
 
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.
-exclude_patterns = ['_build', '_theme']
+exclude_patterns = ['_build', '_theme', 'requirements.txt']
 
 # The reST default role (used for this markup: `text`) to use for all documents.
 # default_role = None