chore(deps): update sphinx requirement from <7.0,>=4.0 to >=4.0,<9.0 (appium#1009)

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.rst)
- [Commits](sphinx-doc/sphinx@v4.0.0...v8.0.2)

---
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>

Author: dependabot[bot]

docs/requirements.txt

@@ -1,2 +1,2 @@
-sphinx >= 4.0, < 7.0
+sphinx >= 4.0, < 9.0
 sphinx_rtd_theme < 3.0