tests and CI for `import-linter`

I am opening this issue to talk about CI and testing approach in this great package. We can start from discussing following points:

1. Why travis is not fully integrated with project? Usually CI systems use github’s checks to give info about build result etc which is really handy - example of github checks with travis
2. What do you say for migrating from travis to github actions and workflows?
3. Is tox really needed? Maybe travis build matrix or github workflows strategy matrix will be enough?
4. What do you think about services like codecov? It’s free for open source projects and can help us to get 100% code coverage and then keep it as this level

[Czaki]

I suggest staying with tox. It simplifies local testing.

[jakub-borusewicz]

Would be great to add mypy check to CI with quite strict configuration (and appropriate typing), it would make contributions a lot easier 😃