Add requirements.txt for old sphinx version used here

Our ReadTheDocs project config now specifies the use of a requirements.txt file
so builds for old branches started failing without it.

requirements.txt

@@ -0,0 +1,8 @@
+# Base dependencies
+
+# Sync with readthedocs:
+# https://github.com/readthedocs/readthedocs.org/blob/master/requirements/pip.txt
+# https://github.com/readthedocs/readthedocs.org/blob/master/requirements/local-docs-build.txt
+sphinx==1.8.5  # pyup: <2.0.0
+sphinx_rtd_theme==0.4.3
+sphinx-notfound-page==0.6