Jonas Obrist

aiodynamo `RangeKey` has an `is_in` method, but that [doesn't seem to be allowed in a KeyConditionExpression](https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_Query.html#DDB-Query-request-KeyConditionExpression).

manually document enums

5

Fixes #190

autodocs for enums generate crazy output

2

see https://aiodynamo.readthedocs.io/en/latest/usage.html#aiodynamo.models.ReturnValues

Adds support for [Process Credentials Provider](https://docs.aws.amazon.com/sdkref/latest/guide/feature-process-credentials.html)