feature gate deprecated APIs for PyList (#4127)

Author: Icxolu

guide/src/migration.md

@@ -75,7 +75,7 @@ Python::with_gil(|py| {
 
 After:
 
-```rust
+```rust,ignore
 # use pyo3::prelude::*;
 # use pyo3::types::{PyInt, PyList};
 # fn main() -> PyResult<()> {
@@ -1089,7 +1089,7 @@ An additional advantage of using Rust's indexing conventions for these types is
 that these types can now also support Rust's indexing operators as part of a
 consistent API:
 
-```rust
+```rust,ignore
 #![allow(deprecated)]
 use pyo3::{Python, types::PyList};
 

guide/src/types.md

@@ -330,8 +330,10 @@ For a `&PyAny` object reference `any` where the underlying object is a Python-na
 a list:
 
 ```rust
+# #![allow(unused_imports)]
 # use pyo3::prelude::*;
 # use pyo3::types::PyList;
+# #[cfg(feature = "gil-refs")]
 # Python::with_gil(|py| -> PyResult<()> {
 #[allow(deprecated)] // PyList::empty is part of the deprecated "GIL Refs" API.
 let obj: &PyAny = PyList::empty(py);
@@ -390,8 +392,10 @@ To see all Python types exposed by `PyO3` consult the [`pyo3::types`][pyo3::type
 **Conversions:**
 
 ```rust
+# #![allow(unused_imports)]
 # use pyo3::prelude::*;
 # use pyo3::types::PyList;
+# #[cfg(feature = "gil-refs")]
 # Python::with_gil(|py| -> PyResult<()> {
 #[allow(deprecated)] // PyList::empty is part of the deprecated "GIL Refs" API.
 let list = PyList::empty(py);