v1.2.1

Changes:

• Added a __init__.py file in the tests folder to simplify running the tests
• Added a CONTRIBUTING.md file with contribution guidelines
• Started using coverage to measure the code's test coverage:
  • Added a .coveragerc configuration file for coverage
  • Added a test coverage badge (docs/coverage-badge.svg), made using coverage-badge
• Added dev optional dependencies for development in pyproject.toml
• Reformatted function definitions of less than 88 characters to take only one line