Avoid TOCTOU errors in .python-version reads (#5223)

## Summary

Not a big deal, but better to try the operation and handle the failure
case than to check if the file exists and _then_ read it.

crates/uv-python/src/lib.rs

@@ -14,8 +14,8 @@ pub use crate::prefix::Prefix;
 pub use crate::python_version::PythonVersion;
 pub use crate::target::Target;
 pub use crate::version_files::{
-    request_from_version_file, requests_from_version_file, version_file_exists,
-    versions_file_exists, PYTHON_VERSIONS_FILENAME, PYTHON_VERSION_FILENAME,
+    request_from_version_file, requests_from_version_file, PYTHON_VERSIONS_FILENAME,
+    PYTHON_VERSION_FILENAME,
 };
 pub use crate::virtualenv::{Error as VirtualEnvError, PyVenvConfiguration, VirtualEnvironment};
 mod discovery;