Feature-gated the crate-level deny(missing_debug_implementations) attribute.

This is necessary since the `pyo3::pyobject_native_type_base` macro only implements `Debug` with the "gil-refs" feature since the 0.22.0 update. The `Debug` impls are no longer relevant anyway because `&PyArray`, `&PyArrayDescr`, etc. can only be obtained through the gil-refs API's.

Author: JRRudy1

src/lib.rs

@@ -69,8 +69,11 @@ as well as the [`PyReadonlyArray::try_as_matrix`] and [`PyReadwriteArray::try_as
 //!
 //! [c-api]: https://numpy.org/doc/stable/reference/c-api
 //! [ndarray]: https://numpy.org/doc/stable/reference/arrays.ndarray.html
-
-#![deny(missing_docs, missing_debug_implementations)]
+#![deny(missing_docs)]
+// requiring `Debug` impls is not relevant without gil-refs since `&PyArray` and 
+// similar aren't constructible and the `pyo3::pyobject_native_type_base` macro 
+// does not provide a `Debug` impl
+#![cfg_attr(feature = "gil-refs", deny(missing_debug_implementations))]
 
 #[cfg(all(target_os = "windows", target_arch = "x86"))]
 compile_error!("Compilation for 32-bit windows is not currently supported. See https://github.com/PyO3/rust-numpy/issues/448");