Pin sphinx <5.1.0 (#565)

requirements/docs.txt

@@ -1,6 +1,6 @@
 # dependencies for building docs, separate from dev.txt as this is also used for builds on readthedocs.org
 # core dependencies
-sphinx>=4.2.0, <6.0.0
+sphinx>=4.2.0, <5.1.0
 sphinx-autodoc-typehints>=1.12.0, <2.0.0
 sphinx-rtd-theme>=1.0.0, <2.0.0
 sphinxcontrib-apidoc>=0.3.0, <0.4.0