build: update sphinx requirement from <7.4,>=5.3.0 to >=5.3.0,<8.1

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/v8.0.2/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v5.3.0...v8.0.2)

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

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

pyproject.toml

@@ -54,7 +54,7 @@ docs = [
   "pybtex",
   "pydata-sphinx-theme>=0.12",
   "setuptools>=61.0.0",
-  "sphinx>=5.3.0,<7.4",
+  "sphinx>=5.3.0,<8.1",
   "sphinx-copybutton>=0.4.0",
   "sphinx-favicon>=1.0.1",
   "sphinx_design>=0.3.0",