Ignore pre-release versions in uv init --script (#15747)

## Summary

Ignore managed pre-release versions of Python for purposes of creating a
`requires-python` constraint when running `uv init --script`. This makes
the behavior consistent with `uv init` for normal projects.

## Test Plan

Added a regression test that makes sure that the constraint is
`requires-python = ">=3.13"`, even though a pre-release version of 3.14
is installed.

.python-versions

@@ -10,3 +10,5 @@
 3.9.12
 # The following is needed for `==3.13` request tests
 3.13.0
+# A pre-release version required for testing
+3.14.0rc2