docs: prevent build error by rolling back Sphinx version

We're hitting a sphinxcontrib-httpdomain vs sphinx issue that was
causing the openapi part of doc builds to error out with:

<openapi>:1:Problem in http domain: field is supposed to use role 'obj', but that role is not in the domain.

See sphinx-contrib/httpdomain#51

Until it's fixed, hold back the Sphinx version to < 4.1.0

Signed-off-by: Daniel Axtens <[email protected]>
Reviewed-by: Stephen Finucane <[email protected]>

Author: daxtens

docs/requirements.txt

@@ -1,4 +1,6 @@
-sphinx>=2.0
+# sphinx 4.1.x breaks sphinxcontrib-httpdomain which sphinxcontrib-openapi depends on
+# see https://github.com/sphinx-contrib/httpdomain/pull/51
+sphinx>=2.0,<4.1
 reno>=2.2
 sphinx_rtd_theme~=0.5.0
 jinja2~=2.11.2