requirements: update sphinx requirement from <4.2.1 to <8.1.4

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@v0.1.61611...v8.1.3)

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

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

doc_requirements.txt

@@ -1,3 +1,3 @@
 psyneulink-sphinx-theme<1.2.4.1
-sphinx<4.2.1
+sphinx<8.1.4
 sphinx_autodoc_typehints<1.16.0