Update of dependencies and support for Python3.10 to Python3.13

[MathisFranel]

Fixes #160.

Changes proposed in this PR:
-Added support for Python3.12 and 3.13
-Removed support for Python3.8 and 3.9
-Replaced down https://mermade.org.uk/ API by https://converter.swagger.io/ for converting from Swagger/OpenAPI 2.0 to OpenAPI 3.0.0
-Replace down http://finkhaeuser.de/projects/prance/petstore.yaml by https://petstore.swagger.io/v2/swagger.yaml
-Replace deprecated RefResolutionError and validate_spec in BaseParser
-Added skipif for flex related tests (+coverage 89 due to more skipped tests) + backend specified for openapi-spec-validator related tests
-Update of project dependencies

The coverage for flex is at 89.5% and at 98.87% for other cases

[MathisFranel]

Hi @RonnyPfannschmidt,
Could you check this one please? It could help on few other projects that rely on your work.
Thanks in advance!

[RonnyPfannschmidt]

Thanks for getting this started

At first glance it looks like ill have to schedule a time block for review

[fwininger]

Thanks @MathisFranel

[RonnyPfannschmidt]

I'm down with a cold since a few days so getting this in currently is delayed

[MathisFranel]

Hi @RonnyPfannschmidt,
Have you had time to check the PR?

[RonnyPfannschmidt]

Nope had a flu that kept me down quite a while and I'm still catching up

[dseichter]

Hope you're getting well soon!
This PR will also fix my open report #162. Within an isolated environment, also the vulnerability scan tool Wapiti, will worked, if this PR will be merged and used for the next upcoming release of prance.