[dev-env] requirements-dev: unpin Sphinx-doc at v7.1.2

- Sphinx-doc 7.2.0 drops py3.8 support [1][2]
- last version with py3.8 support is 7.1.2

Many LTS distributions still have py3.8 which EOL is in 2024-10 [3].

To continue to support a development environment on py3.8 the rigid dependency
in the development environment is unpinned.

[1] searxng#2658 (comment)
[2] sphinx-doc/sphinx#11621
[3] https://devguide.python.org/versions/#supported-versions

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

requirements-dev.txt

@@ -7,7 +7,7 @@ splinter==0.19.0
 selenium==4.11.2
 twine==4.0.2
 Pallets-Sphinx-Themes==2.1.1
-Sphinx==7.1.2
+Sphinx>=7.1.2
 sphinx-issues==3.0.1
 sphinx-jinja==2.0.2
 sphinx-tabs==3.4.1