Integrate coverage in GitLab coverage feature

Todo

• The coverage can reports to the terminal and as htmls altogether:

poetry run pytest --cov silkaj --cov-report html:cov_html --cov-report term

• Switch from pytest-cov to coverage dependency?:

poetry run coverage run -m pytest --cov silkaj --cov-report html:cov_html --cov-report term

• Then, specify coverage regex in the configuration to find it in the logs of the job, to generate the badge and a chart of the coverage evolution, which is available from GitLab v13.1.
• Remove now useless coverage-badge dependency introduced in #146 (closed).
• Update badge in the readme and the repository configuration