Fix RTD builds.

Michael Howitz · Michael Howitz · commit c1a3e7ca64cd · 2021-11-11T08:14:03.000+01:00
Same approach which helped for zope.sequencesort.
diff --git a/.readthedocs.yaml b/.readthedocs.yaml
@@ -0,0 +1,5 @@
+version: 2
+
+python:
+  install:
+  - requirements: docs/requirements.txt
diff --git a/docs/requirements.txt b/docs/requirements.txt
@@ -1,3 +1,4 @@
-Zope
+Sphinx==4.2.0
 ZConfig
+Zope
 tempstorage
