[CI] drop Python 3.7 from CI

- Python 3.7 supports security fixes only, as needed, until 2023-06 [1]
- Some of SearXNG's dependencies do no longer support Python 3.7 [2]

[1] https://peps.python.org/pep-0537/#and-beyond-schedule
[2] https://github.com/searxng/searxng/pull/2102



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