This codebase is using standard python unittest framework

pytest is arguably more advanced and defacto standard for concise unit testing.

• fixture based testing (tempfile, etc)
• module function based testing without the need to define class
• pytest.parametrized is also very useful (to me)

I would propose to include pytest as a dev dependency, so that it is easier to write tests for spdx/tools-python

We can keep the tests as they are right now, we don't need to translate them as pytest also support standard test classes.

I'll do the PR, if maintainers are agreeing to the change.