Fix optional cfg gates (#14448)

Running `cargo clippy` in individual crates could raise warnings due to
unused imports as `Cow` is only used with `#[cfg(feature = "schemars")]`

crates/uv-python/src/python_version.rs

@@ -1,3 +1,4 @@
+#[cfg(feature = "schemars")]
 use std::borrow::Cow;
 use std::fmt::{Display, Formatter};
 use std::ops::Deref;