Update sphinx requirement from <7,>=6 to >=6,<8 (#1280)

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@v6.0.0...v7.0.0)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements_readthedocs.txt

@@ -3,5 +3,5 @@
 # dependency expressions (>=1.0b1 failes to find dist)
 Django>=3.1.13
 bleach>=3.3.0,<6.1
-sphinx>=6,<7
+sphinx>=6,<8
 sphinx_rtd_theme==1.2.0