💚 Migrate CI from travis to gh-actions

[azogue]

As travis.ci is not working anymore 😢, the day has come to move to the new 😅 GitHub Actions.

The desired flow is the same as before:

• to run lint + test stages for all commits in PRs and master branch,
• and publish new versions based on changes in the pyproject file, when merging to master.

Changes:

• 🎨 Add isort to pre-commit config
• 💚 Add configuration for pre-commit.ci to run linter checks there
• 💚 CI flow with GitHub Actions to
  • install library with poetry
  • run tests
  • upload coverage when merging to master
  • publish a new pypi version when merging to master if pyproject.toml changes
• 🔥 Remove previus travis CI config