Rename Error::IO to Error::Io (#5174)

## Summary

I believe this is by convention (see, e.g., in Rust itself:
https://github.com/search?q=repo%3Arust-lang%2Frust+%2F%28%3F-i%29Io%2F&type=code).

crates/uv-python/src/virtualenv.rs

@@ -32,12 +32,12 @@ pub struct PyVenvConfiguration {
 
 #[derive(Debug, Error)]
 pub enum Error {
+    #[error(transparent)]
+    Io(#[from] io::Error),
     #[error("Broken virtualenv `{0}`: `pyvenv.cfg` is missing")]
     MissingPyVenvCfg(PathBuf),
     #[error("Broken virtualenv `{0}`: `pyvenv.cfg` could not be parsed")]
     ParsePyVenvCfg(PathBuf, #[source] io::Error),
-    #[error(transparent)]
-    IO(#[from] io::Error),
 }
 
 /// Locate an active virtual environment by inspecting environment variables.