Disallow sphinx>=4.0 for now: it's breaking on CI

Exception occurred:
  File /usr/lib/python3/dist-packages/docutils/nodes.py, line 625, in __getitem__
    return self.attributes[key]
KeyError: 'domain'

diff --git a/docs/requirements.txt b/docs/requirements.txt
index 401e2dc..4466c18 100644 (file)
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -1,5 +1,5 @@
 breathe
-sphinx>=3.4.3
+sphinx>=3.4.3,<4.0
 sphinx_rtd_theme
 # sphinx_tabs v1.2.1 is required for Sphinx 2
 sphinx_tabs>=1.2.1