Add integration with Travis CI

[webknjaz]

Please configure integration with TravisCI:

• [ ] Enable repo testing in TravisCI
• [ ] Add TravisCI badge (master branch) to README.md with a single commit. (click next to octocat icon on this page)
• [ ] Create new branch called feature/travis-ci-integration and do all the following steps within that branch.
• [ ] Add .travis.yml using pre-commit as a script. Set it up to run Python 3.8 env and add pip caching. Ensure you install pre-commit before running tests:

  language: python
  python: 3.8
  cache: pip
  before_install: pip install -U pre-commit
  script: pre-commit run --all-files

• [ ] Push the branch and create a Pull Request for it.