Ignore Python 2 installations when querying for interpreters (#1905)

## Summary

Fixes https://github.com/astral-sh/uv/issues/1693

`uv` currently fails when a user has `python` 2 or older installed on
their system without a `python3` or `python3.exe` on their path because
the `get_interpreter_info.py` script fails executing (it uses some
Python 3+ APIs).

This PR fixes this by:

* Returning an explicit error code in `get_interpreter_info` if the
Python version isn't supported
* Skipping over this error in `python_query` if the user requested ANY
python version or a version >= 3.
* Error if the user requested a Python 2 version. 

## Test Plan

Error if the user requests a legacy python version. 

```
uv venv -p 2
  × Python 2 or older is not supported. Please use Python 3 or newer.
```


Ignore any python 2 installation when querying newer python
installations (using v4 here because I have python3 on the path and that
takes precedence over querying python)
```
 uv_interpreter::python_query::find_python selector=Major(4)
      0.005541s   0ms DEBUG uv_interpreter::interpreter Detecting markers for: /home/micha/.pyenv/shims/python
      0.059730s  54ms DEBUG uv_interpreter::python_query Found a Python 2 installation that isn't supported by uv, skipping.
      0.059983s  54ms DEBUG uv_interpreter::interpreter Using cached markers for: /usr/bin/python
  × No Python 4 In `PATH`. Is Python 4 installed?

```

crates/uv-interpreter/src/get_interpreter_info.py

@@ -1,3 +1,13 @@
+""""
+Queries information about the current Python interpreter and prints it as JSON.
+
+Exit Codes:
+    0: Success
+    1: General failure
+    3: Python version 3 or newer is required
+"""
+
+
 import json
 import os
 import platform
@@ -13,6 +23,10 @@ def format_full_version(info):
     return version
 
 
+if sys.version_info[0] < 3:
+    sys.exit(3)
+
+
 if hasattr(sys, "implementation"):
     implementation_version = format_full_version(sys.implementation.version)
     implementation_name = sys.implementation.name

crates/uv-interpreter/src/interpreter.rs

@@ -36,7 +36,7 @@ pub struct Interpreter {
 impl Interpreter {
     /// Detect the interpreter info for the given Python executable.
     pub fn query(executable: &Path, platform: &Platform, cache: &Cache) -> Result<Self, Error> {
-        let info = InterpreterQueryResult::query_cached(executable, cache)?;
+        let info = InterpreterInfo::query_cached(executable, cache)?;
 
         debug_assert!(
             info.sys_executable.is_absolute(),
@@ -290,7 +290,7 @@ impl Interpreter {
 }
 
 #[derive(Debug, Deserialize, Serialize, Clone)]
-pub(crate) struct InterpreterQueryResult {
+pub(crate) struct InterpreterInfo {
     pub(crate) markers: MarkerEnvironment,
     pub(crate) base_exec_prefix: PathBuf,
     pub(crate) base_prefix: PathBuf,
@@ -298,8 +298,8 @@ pub(crate) struct InterpreterQueryResult {
     pub(crate) sys_executable: PathBuf,
 }
 
-impl InterpreterQueryResult {
-    /// Return the resolved [`InterpreterQueryResult`] for the given Python executable.
+impl InterpreterInfo {
+    /// Return the resolved [`InterpreterInfo`] for the given Python executable.
     pub(crate) fn query(interpreter: &Path) -> Result<Self, Error> {
         let script = include_str!("get_interpreter_info.py");
         let output = if cfg!(windows)
@@ -349,6 +349,10 @@ impl InterpreterQueryResult {
         // stderr isn't technically a criterion for success, but i don't know of any cases where there
         // should be stderr output and if there is, we want to know
         if !output.status.success() || !output.stderr.is_empty() {
+            if output.status.code() == Some(3) {
+                return Err(Error::Python2OrOlder);
+            }
+
             return Err(Error::PythonSubcommandOutput {
                 message: format!(
                     "Querying Python at `{}` failed with status {}",
@@ -359,7 +363,8 @@ impl InterpreterQueryResult {
                 stderr: String::from_utf8_lossy(&output.stderr).trim().to_string(),
             });
         }
-        let data = serde_json::from_slice::<Self>(&output.stdout).map_err(|err| {
+
+        let data: Self = serde_json::from_slice(&output.stdout).map_err(|err| {
             Error::PythonSubcommandOutput {
                 message: format!(
                     "Querying Python at `{}` did not return the expected data: {err}",

crates/uv-interpreter/src/lib.rs

@@ -59,6 +59,8 @@ pub enum Error {
         stdout: String,
         stderr: String,
     },
+    #[error("Python 2 or older is not supported. Please use Python 3 or newer.")]
+    Python2OrOlder,
     #[error("Failed to write to cache")]
     Encode(#[from] rmp_serde::encode::Error),
     #[error("Broken virtualenv: Failed to parse pyvenv.cfg")]

crates/uv-interpreter/src/python_query.rs

@@ -146,9 +146,20 @@ fn find_python(
                         continue;
                     }
 
-                    let installation = PythonInstallation::Interpreter(Interpreter::query(
-                        &path, platform, cache,
-                    )?);
+                    let interpreter = match Interpreter::query(&path, platform, cache) {
+                        Ok(interpreter) => interpreter,
+                        Err(Error::Python2OrOlder) => {
+                            if selector.major() <= Some(2) {
+                                return Err(Error::Python2OrOlder);
+                            }
+                            // Skip over Python 2 or older installation when querying for a recent python installation.
+                            tracing::debug!("Found a Python 2 installation that isn't supported by uv, skipping.");
+                            continue;
+                        }
+                        Err(error) => return Err(error),
+                    };
+
+                    let installation = PythonInstallation::Interpreter(interpreter);
 
                     if let Some(interpreter) = installation.select(selector, platform, cache)? {
                         return Ok(Some(interpreter));
@@ -306,6 +317,15 @@ impl PythonVersionSelector {
             ],
         }
     }
+
+    fn major(self) -> Option<u8> {
+        match self {
+            PythonVersionSelector::Default => None,
+            PythonVersionSelector::Major(major) => Some(major),
+            PythonVersionSelector::MajorMinor(major, _) => Some(major),
+            PythonVersionSelector::MajorMinorPatch(major, _, _) => Some(major),
+        }
+    }
 }
 
 mod windows {