CI: move to pytest+codecov

Currently devolearn uses a relatively simple testing suite i.e `python3 setup.py test`. But a much cleaner alternative would be to use `pytest` for testing and `codecov` for coverage reports.

**Why use coverage reports ?**
They help us see how much of the code is being tested after each push, this'll help us find the pieces of code that are important but remain untested.

**Why pytest ?**
From my own personal experience, `python3 setup.py test` crashes **without** an error message when the user tries to access a CUDA device (GPU) on the github actions runtime. This error would've been impossible to fix if I hadn't moved to using pytest (which showed me the proper error message).