Update sphinx requirement from <7 to <8 (#256)

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@v0.1.61611...v7.0.0)

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

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

Author: dependabot[bot]

setup.cfg

@@ -42,7 +42,7 @@ dev =
     pytest-xdist<4
     responses==0.23.1
 docs =
-    sphinx<7
+    sphinx<8
     sphinx-issues<4
     furo==2023.3.27