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The main goal of this release is the migration to `poetry` and stronger type
checking with mypy.
User code should remain compatible, just test it with mypy. The tests still
work the same, without public API changes, this indicates that your code
should work without changes, but does not prove it.
Deprecations
* `aio_pika.message.HeaderProxy` - removed
* `aio_pika.message.header_converter` - removed
* `aio_pika.message.format_headers` - removed
* `aio_pika.message.Message.headers_raw` - prints deprecation warning
* `aio_pika.abc.AbstractMessage.headers_raw` - removed