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`).
