Revert sphinx version to <6.0.0 (#708)

* Revert "Remove `requirements/docs.txt` comment"

This reverts commit 1c9942d.

* Revert "Update sphinx-autodoc-typehints requirement (#699)"

This reverts commit 87fa4ef.

* Revert "Update sphinx requirement from <5.1.0,>=4.2.0 to >=4.2.0,<6.1.0 (#696)"

This reverts commit a893f51.

* Bump sphinx to <6.0.0

Author: Ashley Scillitoe

requirements/docs.txt

@@ -1,7 +1,7 @@
 # dependencies for building docs, separate from dev.txt as this is also used for builds on readthedocs.org
 # core dependencies
-sphinx>=4.2.0, <6.1.0
-sphinx-autodoc-typehints>=1.12.0, <1.20.1
+sphinx>=4.2.0, <6.0.0
+sphinx-autodoc-typehints>=1.12.0, <1.19.3  # limited due to https://github.com/tox-dev/sphinx-autodoc-typehints/issues/259 and 260 
 sphinx-rtd-theme>=1.0.0, <2.0.0
 sphinxcontrib-apidoc>=0.3.0, <0.4.0
 sphinxcontrib-bibtex>=2.1.0, <3.0.0