Update sphinx requirement from <6.0.0,>=4.2.0 to >=4.2.0,<7.0.0

Updates the requirements on [sphinx](https://github.com/sphinx-doc/sphinx) to permit the latest version.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES)
- [Commits](sphinx-doc/sphinx@v4.2.0...v6.1.3)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

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, <7.0.0
 sphinx-autodoc-typehints>=1.12.0, <1.21  # limited due to https://github.com/tox-dev/sphinx-autodoc-typehints/issues/260
 sphinx-rtd-theme>=1.0.0, <2.0.0
 myst-parser>=0.14, <0.19