Dotenv-cli

* on POSIX systems we don't fork a new child process anymore but use `exec*`
to replace the `dotenv` process
* Dropped Python 3.7 support
* replaced setup.py/.cfg with pyproject.toml
* modernized github actions:
* don't run linter and mypy on all platforms, only one
* run test-release
* updated dev-dependencies

* updated dependencies:
* mypy
* pytest
* pytest-cov
* wheel
* Debian:
* added htmlcov and .mypy_cache to extended-diff-ignore
* bump debhelper from 11 -> 13
* use debhelper-compat
* use standards-version 4.6.2

* added type hints and mypy --strict to test suite
* updated dependencies:
* flake8
* pytest
* twine

* bumped version (no changes)

* removed python 3.6 support
* added dependabot
* updated makefile

* 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