Update pydata-sphinx-theme requirement

Updates the requirements on [pydata-sphinx-theme](https://github.com/pydata/pydata-sphinx-theme) to permit the latest version.
- [Release notes](https://github.com/pydata/pydata-sphinx-theme/releases)
- [Changelog](https://github.com/pydata/pydata-sphinx-theme/blob/main/RELEASE.md)
- [Commits](pydata/pydata-sphinx-theme@v0.6.0...v0.15.2)

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

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

docs/sphinx-requirements.txt

@@ -5,7 +5,7 @@ sphinx>=3.0,<7.3
 Pygments>=2.6.1
 
 # Sphinx extensions
-pydata-sphinx-theme>=0.6.0,<0.8.1
+pydata-sphinx-theme>=0.6.0,<0.15.3
 sphinx-automodapi
 sphinx-gallery