Warn user on startup if `requires-python` is not met

[gy-mate]

I think that zipapps generated with shiv should warn users on start if the requires-python Trove classifier defined in pyproject.toml is not met by the user's Python environment. The warning might just be something like this:

shiv: Your Python version (x.xx) does not meet the zipapp's requirements (<>= x.xx). Please upgrade/downgrade it!
Do you want to continue anyway? The app might not behave as intended. (y/n)

See gy-mate/moodle-to-vikwikiquiz#17 for a user report.