* Allow for missing .env file -- in this case the command will be executed
without setting any environment variables. The previous behaviour was to
fail with a FileNotFoundError
* Migrated from TravisCI to github actions. We test now on Linux, Mac and
Windows x all supported Python versions!
* Fixed tests under windows, where NamedTemporaryFile cannot be opened twice.
* refactored __main__.py into cli.py and wrapped argparsing into dedicated
function
* bumped minimal Python version to 3.6
* Added 3.8, 3.9 to travis tests
* Cleaned up Makefile
* Added twine to dev-dependencies