python -m turtle_challenges <challenge> and the turtle-challenges command open a
window, run the named challenge and wait for a click. square and dashed-line are
registered; a missing Tk is reported with a hint instead of a traceback.
Task: MIL-002#6
Requirements, set-up for Windows PowerShell, Linux Debian and macOS with a local
.venv and an upgraded pip, how to run the tests, continuous integration, the
source documentation, the project layout and the licence. The Run section is
completed with the last milestone.
Task: MIL-001#6