Add task runners for tests and extend contributing docs

[matthewfeickert]

I would like to have the skip so that the tests do not fail if setuptools is not installed (for new developers this could be frustrating). If you do not have time to do it let us just open an issue and merge the current PR

Originally posted by @rflamary in https://github.com/PythonOT/POT/issues/635#issuecomment-2182996937

Conditional skipping of tests based on a developer's environment isn't great, and a better solution is to use a task runner like nox to ensure that the environment is automatically getting configured correctly. Additionally, extending the developer docs (https://pythonot.github.io/contributing.html#how-to-contribute) to include environment setup instructions would help.