Fix CI test and adapt to new version of dependencies

[darrencl]

Fixing CI:

• Points pytest to tests/ directory instead of tests.py
• Maintain requirements_dev.txt to add extra dependencies for testing purpose, e.g., pytest.
• Adapt code to new version of dependencies

Closes #162, #167

[darrencl]

Hi @bennyrowland ,

Here is the PR that fixed the initial CI and adapts the code to work in newer versions of dependencies. My goal here is to check that everything works as before. I would make a separate PR If there is any refactor to tidy up the code related to this changes, so that the master branch could benefit on this changes ASAP.

Thanks!

[bennyrowland]

@darrencl, thanks for this, it all looks fine to me - I just had one question that is more for my own understanding than anything, I am happy for you to merge it in.