cap Sphinx version at 6.x

Due to an incompatibility between sphinx-doc/sphinx#11381
and the current Read The Docs theme

See readthedocs/sphinx_rtd_theme#1464

Author: mr-c

docs/requirements.txt

@@ -1,4 +1,4 @@
-sphinx >= 2.2
+sphinx >= 2.2, <7
 sphinx-rtd-theme
 sphinx-autoapi
 sphinx-autodoc-typehints