Switching CI to GitHub actions

[hleb-rubanau]

This pull request reproduces previous Travis workflow on GithubActions with some tweaks & enhancements.

Key features:

• Workflow is named "CI & Release"
• Separate jobs: linter, test, smoketest, release
• Linter failure is tolerable
• Smoketest tries dockerhub login for better pulling but can proceed without it
• Release is only activated for selected branches (currently: master)
• If release happens outside of master e.g. during debug on ci/**, artifacts are pushed to testpypi instead of main repo (condition is configurable)

Examples:

• Development branch (not to be merged!) demonstrates release pipeline in action
• Same dev pipeline published package on testpypi
• Tags created by dev pipeline -- unfortunately, tag space pollution occured (from 2.1.9 to 2.1.15). Maybe for the future semantic-release should use conditional suffixes on non-master version tags (if such logic is supported)

After review and before merging:

• Delete tags created by dev pipeline starting from 2.1.9 to 2.1.15 (TODO: maybe use custom suffix for test runs of pipeline)
• Setup secrets:
  • POETRY_PYPI_TOKEN_PYPI (pypi user -- https://pypi.org/user/applandrobot/ )
  • DOCKERHUB_PASSWORD for applandinc user (this is optional but nice-to-have)
• (recommended) disable triggers for the repo on Travis-ci.com