release_notes: document more breaking changes (pynamodb#1222)

Bump version to 6.0.0 (pynamodb#1216)

Fix compatibility with botocore 1.33.2 (pynamodb#1206)

Add 'add_version_condition' arg (pynamodb#1177)

Add a flag for controlling whether `Model.save`, `Model.update` and `Model.delete` add a condition that the persisted version is the same as the in-memory one, defaulting to `True` (the "safe" behavior).

Use model's AWS credentials in threads

When a model specifies custom AWS credentials (instead of global ones), they should be used when creating new sessions in threads. Previously, threads would always use the global credentials.

Expose transaction cancellation reasons (pynamodb#1156)

This will allow the caller to know e.g. which of the transaction items ran into a conflict, which can sometimes allow app logic to better handle the error without needing to make more requests.

Fix exception message (pynamodb#1157)

5.x: propagate null_check to maps in lists (pynamodb#1128)

When calling `model.serialize(null_check=False)`, we were propagating `null_check=False` to maps (and maps nested in maps), but not maps nested in lists.

Backport of pynamodb#1127 to 5.x branch.

4.x: workaround for _convert_to_request_dict change (pynamodb#1083) (p…

…ynamodb#1130)

botocore 1.28 changed the signature of private method botocore.client.BaseClient._convert_to_request_dict adding an endpoint_url parameter. We are updating pynamodb to inspect the signature and add this parameter as needed.

Improve first iteration logic (pynamodb#1121)

Backporting pynamodb#1101 to 5.x branch to allow handling an exception raised when retrieving the first item.