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

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 https://github.com/gy-mate/moodle-to-vikwikiquiz/issues/17 for a user report._

The issue concerns shiv-generated zipapps, the requires-python value in pyproject.toml, and startup behavior; begin by tracing the generated app's startup entry point and how this requirement is carried into it. Confirm the Python-version comparison and warning/continue prompt using the reported moodle-to-vikwikiquiz case, with completion defined as warning when the requirement is unmet.

Written by the indexing model from the issue text.