Dotenv-cli

* added option to completely replace environment with contents of the dotenv
file
* added very simple docs for readthedocs

* replaced flake8 with ruff
* when building the debian package, don't run coverage when running tests

* updated debian/watch
* updated dev-dependencies

* 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