Add simplified rye fetch

[mitsuhiko]

This improves the behavior of rye fetch. When invoked without arguments it will now try to fetch the version of the requested Python interpreter. Specifically this makes the following command work even without an installed Python interpreter without manually having to copy paste anything:

rye pin 3.8
rye fetch
python

The help message is now also adjusted. Additionally this fixes an issue where the .python-version created by the rye init command would freeze a to specific version request (usually with architecture). Behavior is now aligned with rye pin.