Feat/package updates - Githubissues

This PR makes a lot of changes but they are grouped by commits:

updated Readme according to https://github.com/sentinel-hub/sentinel2-cloud-detector/issues/36,
fixed a deprecation warning raised by sentinelhub-py,
applied black and isort formatters,
removed support for Python 3.6, added support for 3.9 and 3.10,
switched from Travis CI to GitHub actions,
upgraded release procedure to also produce a wheel.

sentinel-hub / sentinel2-cloud-detector