Make PyDict iterator compatible with free-threaded build (#4439)

* Iterate over dict items in `DictIterator`

* Use python instead of C API to get dict items

* Use plain dict iterator when not on free-threaded & not a subclass

* Copy dict on free-threaded builds to prevent concurrent modifications

* Add test for dict subclass iters

* Implement `PyDict::locked_for_each`

* Lock `BoundDictIterator::next` on each iteration

* Implement locked `fold` & `try_fold`

* Implement `all`,`any`,`find`,`find_map`,`position` when not on nightly

* Add changelog

* Use critical section wrapper

* Make `dict::locked_for_each` available on all builds

* Remove item() iter

* Add tests for `PyDict::iter()` reducers

* Add more docs to locked_for_each

* Move iter implementation into inner struct

newsfragments/4439.changed.md

@@ -0,0 +1,3 @@
+* Make `PyDict` iterator compatible with free-threaded build
+* Added `PyDict::locked_for_each` method to iterate on free-threaded builds to prevent the dict being mutated during iteration
+* Iterate over `dict.items()` when dict is subclassed from `PyDict`

src/lib.rs

@@ -1,5 +1,8 @@
 #![warn(missing_docs)]
-#![cfg_attr(feature = "nightly", feature(auto_traits, negative_impls))]
+#![cfg_attr(
+    feature = "nightly",
+    feature(auto_traits, negative_impls, try_trait_v2)
+)]
 #![cfg_attr(docsrs, feature(doc_cfg, doc_auto_cfg))]
 // Deny some lints in doctests.
 // Use `#[allow(...)]` locally to override.