Add CI with lint & unit tests

This brings CI checks on every commit (merge requests & master):

• pre-commit usual checks and linting using black
• tox to run your unit tests (using pytest for execution)

I made a few changes:

• I disabled the formatter around the tests payload that had a nicer structure without black formatting (in #fmt: off/on blocks)
• I removed the pandas dependency as it was unused (reported by flake8 & isort)
• I updated the .bio paths in the unit tests, prefixing them with tests/ as tox runs from the root of the project
• I moved the pytest dependency to tox.ini (it's more a convention than anything)