Use tox for testing

[BenjaminRodenberg]

We currently use python3 setup.py test which is already deprecated for quite some time. I, therefore, get the following warning when running tests via python3 setup.py test:

WARNING: Testing via this command is deprecated and will be removed in a future version. Users looking for a generic test entry point independent of test runner are encouraged to use tox.

[BenjaminRodenberg]

I will probably start working on this issue with a student visiting our chair end of March

[BenjaminRodenberg]

Note that for newer versions of setuptools the command python3 setup.py test actually is not supported anymore. After installing setuptools==72 via pip3 install setuptools==72 I get the following:

fenics-adapter$ python3 setup.py test
...
error: invalid command 'test'

Downgrading to setuptools==71.1.0 via pip3 install setuptools==71.1.0 fixes the issue and python3 setup.py test works as expected.

This clearly shows that we should move from python3 setup.py test to something more modern.