build: Specify minimum required python version

* pyproject.toml: Specify minimum required python version, also drop
classifiers for old python versions.

Python 2 was dropped previously,
See-Commit 5fe5e7c5aa75509dd9d983a9bf0da3d3f09a9b53.

• Signed-off-by: Lucas Hoffmann <lucc@posteo.de>