Pin Sphinx version for documentation

diff --git a/docs/requirements.txt b/docs/requirements.txt
index 4ec2533a66c1306846d2b0b62229c7a0b69d7e21..f5b13cd3a44184a7ffeee7cfd8c192ff7c6ce25e 100644 (file)
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -1,2 +1,3 @@
 recommonmark==0.7.1
+sphinx==4.0.1
 sphinxcontrib-trio==1.1.2