Dynamo3

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* Removed the legacy API (scan, query, update_item, delete_item, put_item, get_item)
* Renamed the new API methods to match the old ones (e.g. scan2 -> scan, query2 -> query)
* Moved constant values into ``dynamo3.constants``. This is where you can now find STRING, BINARY, etc
* Added mypy typing where possible
* Drop support for Python 2
* Add support for table billing mode (aka on-demand tables)
* Add support for SSE, TTL, and transactions

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* Fixed DynamoDB Local link in testing framework

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* Feature: Result objects from get_item have an ``exists`` flag
* Feature: ``wait`` keyword for create and delete table

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* Bug fix: Scans/Queries could return incomplete results if AWS returned an empty Items list

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* New ``RateLimit`` class to avoid blowing through your provisioned throughput

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* New ``Limit`` class for more complex query limit behavior
* Bug fix: Scan and Query with ``Select='COUNT'`` will page results properly