docs: build with sphinx < 2.1

sphinx 2.1 adds an ":async:" option for py:method:: and py:function::
and it does not seem to interact well with sphinxcontrib-asyncio

diff --git a/docs/requirements.txt b/docs/requirements.txt
index 97528d970849f9371ce87d88f02100fb85b4e99d..852493572bb31e6c3cad602e4dc05e44024675be 100644 (file)
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -1,4 +1,4 @@
-sphinx>1.8.2
+sphinx>1.8.2,<2.1
 sphinxcontrib-asyncio
 sphinx_rtd_theme
 Twisted