[dev-env] upgrade Sphinx-doc 7.2.3 and unpin at v7.1.2 on py3.8

- 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 in py3.8 / environment markers [4].

To get 7.2.3. work, a fix in sphinx-notfound-page is needed [5][6].

[1] https://github.com/searxng/searxng/pull/2658#issuecomment-1684867270
[2] https://github.com/sphinx-doc/sphinx/issues/11621
[3] https://devguide.python.org/versions/#supported-versions
[4] https://peps.python.org/pep-0508/#environment-markers
[5] https://github.com/readthedocs/sphinx-notfound-page/issues/219
[6] https://github.com/readthedocs/sphinx-notfound-page/issues/219#issuecomment-1694691135



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