Update sphinx from 1.8.0 to 1.8.1

requirements/docs.txt

@@ -9,9 +9,9 @@ Pygments==2.2.0 \
     --hash=sha256:78f3f434bcc5d6ee09020f92ba487f95ba50f1e3ef83ae96b9d5ffa1bab25c5d \
     --hash=sha256:dbae1046def0efb574852fab9e90209b23f556367b5a320c0bcb871c77c3e8cc
 # Sphinx is required by sphinxcontrib-httpdomain
-Sphinx==1.8.0 \
-    --hash=sha256:c091dbdd5cc5aac6eb95d591a819fd18bccec90ffb048ec465b165a48b839b45 \
-    --hash=sha256:95acd6648902333647a0e0564abdb28a74b0a76d2333148aa35e5ed1f56d3c4b
+Sphinx==1.8.1 \
+    --hash=sha256:d222626d8356de702431e813a05c68a35967e3d66c6cd1c2c89539bb179a7464 \
+    --hash=sha256:652eb8c566f18823a022bb4b6dbc868d366df332a11a0226b5bc3a798a479f17
 # alabaster is required by Sphinx
 alabaster==0.7.11 \
     --hash=sha256:674bb3bab080f598371f4443c5008cbfeb1a5e622dd312395d2d82af2c54c456 \