Update docs.txt (#4492)

requirements/docs.txt

@@ -40,6 +40,10 @@ pytz==2016.10 \
     --hash=sha256:7016b2c4fa075c564b81c37a252a5fccf60d8964aa31b7f5eae59aeb594ae02b \
     --hash=sha256:9a43e20aa537cfad8fe7a1715165c91cb4a6935d40947f2d070e4c80f2dcd22b \
     --hash=sha256:aafbf066975fe217ed49d7d197b26903d3b43e9ca2aa6ba0a211081f13c41917
+# requests is required by Sphinx
+requests==2.13.0 \
+    --hash=sha256:1a720e8862a41aa22e339373b526f508ef0c8988baf48b84d3fc891a8e237efb \
+    --hash=sha256:5722cd09762faa01276230270ff16af7acf7c5c45d623868d9ba116f15791ce8
 # six is required by Sphinx, bleach, cryptography, django-environ, django-extensions, django-tables2, elasticsearch-dsl, html5lib, mock, nobot, pathlib2, prompt-toolkit, pyOpenSSL, sphinxcontrib-httpdomain, traitlets
 six==1.10.0 \
     --hash=sha256:0ff78c403d9bccf5a425a6d31a12aa6b47f1c21ca4dc2573a7e2f32a97335eb1 \