Pynamodb

This is a major release with breaking changes.

Please read the [release notes](https://pynamodb.readthedocs.io/en/latest/release_notes.html#v6-0-0) carefully for a full list of changes.

What's Changed
* Fix compatibility with botocore 1.33.2 by ikonst in https://github.com/pynamodb/PynamoDB/pull/1206


**Full Changelog**: https://github.com/pynamodb/PynamoDB/compare/5.5.0...5.5.1

What's Changed
* Add 'add_version_condition' arg by ikonst in https://github.com/pynamodb/PynamoDB/pull/1177


**Full Changelog**: https://github.com/pynamodb/PynamoDB/compare/5.4.1...5.5.0

What's Changed
* Use model's AWS credentials in threads by atsuoishimoto

**Full Changelog**: https://github.com/pynamodb/PynamoDB/compare/5.4.0...5.4.1

What's Changed
* Expose transaction cancellation reasons by ikonst in https://github.com/pynamodb/PynamoDB/pull/1156


**Full Changelog**: https://github.com/pynamodb/PynamoDB/compare/5.3.5...5.4.0

What's Changed
* Fix exception message by ikonst, pauliokas in https://github.com/pynamodb/PynamoDB/pull/1157


**Full Changelog**: https://github.com/pynamodb/PynamoDB/compare/5.3.4...5.3.5