Search for all python3.x in PATH (#5148)

Search for all `python3.x` minor versions in PATH, skipping those we
already know we can use.

For example, let's say `python` and `python3` are Python 3.10. When a
user requests `>= 3.11`, we still need to find a `python3.12` in PATH.
We do so with a regex matcher.

Fixes #4709

crates/uv-python/Cargo.toml

@@ -52,8 +52,12 @@ tracing = { workspace = true }
 url = { workspace = true }
 which = { workspace = true }
 
+[target.'cfg(any(unix, target_os = "wasi", target_os = "redox"))'.dependencies]
+rustix = { workspace = true }
+
 [target.'cfg(target_os = "windows")'.dependencies]
 winapi = { workspace = true }
+winsafe = { workspace = true }
 
 [dev-dependencies]
 anyhow = { version = "1.0.80" }