Skip Python 2 versions when locating Python (#3476)

## Summary

Unfortunately, the `-I` flag was added in Python 3.4. So if we query a
Python version prior to 3.4 (e.g., Python 2.7), we can't run our script
at all, and lose the ability to match against our structured error.

This PR adds an additional check against the stderr output for these
cases.

Closes https://github.com/astral-sh/uv/issues/3474.

## Test Plan

Installed Python 2.7, and verified that it was skipped (and that we
instead found my `python3`).

crates/uv-interpreter/src/lib.rs

@@ -74,6 +74,8 @@ pub enum Error {
         stdout: String,
         stderr: String,
     },
+    #[error("Requested Python version ({0}) is unsupported")]
+    UnsupportedPython(String),
     #[error("Failed to write to cache")]
     Encode(#[from] rmp_serde::encode::Error),
     #[error("Broken virtualenv: Failed to parse pyvenv.cfg")]