add clippy lints to prepare for no_std support

Author: Person-93

src/lib.rs

@@ -1,3 +1,8 @@
+#![warn(
+    clippy::alloc_instead_of_core,
+    clippy::std_instead_of_alloc,
+    clippy::std_instead_of_core
+)]
 #![warn(missing_docs)]
 #![cfg_attr(
     feature = "nightly",
@@ -336,6 +341,9 @@
 //! [Rust from Python]: https://github.com/PyO3/pyo3#using-rust-from-python
 #![doc = concat!("[Features chapter of the guide]: https://pyo3.rs/v", env!("CARGO_PKG_VERSION"), "/features.html#features-reference \"Features Reference - PyO3 user guide\"")]
 //! [`Ungil`]: crate::marker::Ungil
+
+extern crate alloc;
+
 pub use crate::class::*;
 pub use crate::conversion::{FromPyObject, IntoPyObject, IntoPyObjectExt};
 pub use crate::err::{CastError, CastIntoError, PyErr, PyErrArguments, PyResult, ToPyErr};