Introduce Pytest

Use Pytest test framework instead of unittest, which is a standard Python library.

Advantages

• Explanation by cebash
• #90 (closed): Coverage badge with coverage-badge
• Coverage static web page
• …

Tasks

• Install pytest, pytest-sugar, (pytest-cov)
• Update Makefile rule and .gitlab-ci.yml
• Activate tests/helpers/ws2p.py tests by prefixing the file with test_.
• Update documentation