Pamqp

- Update to support Python 3.6+ only
- Add typing annotations to all modules, callables, and classes
- Moved exceptions from `pamqp.specification` to `pamqp.exceptions`
- Moved constants from `pamqp.specification` to `pamqp.constants`
- Moved base classes out of `pamqp.specification` to `pamqp.base`
- Changed the structure of nested classes for AMQP Commands (Classes & Methods) in `pamqp.specification` to functions in `pamqp.commands`
- Renamed `pamqp.specification.ERRORS` to `pamqp.exceptions.CLASS_MAPPING`
- Remove convenience exports of `pamqp.headers.ContentHeader` and `pamqp.header.ProtocolHeader`
- pamqp.body.ContentBody.value now only supports `bytes`
- Changed `pamqp.decode.timestamp` to return a `datetime.datetime` instance instead of `time.struct_time`.
- Updated `pamqp.encode.support_deprecated_rabbitmq()` to allow for toggling support.
- Changed `pamqp.encode.timestamp` to only support `datetime.datetime` and `time.struct_time` values, dropping epoch (`int`) support.
- Removed `pamqp.frame.BasicProperties.to_dict()` in favor of behavior allowing for `dict(pamqp.frame.BasicProperties)`
- Optimized `pamqp.heartbeat.Heartbeat` to marshal the static frame value as a predefined class attribute.
- Add support for `Connection.UpdateSecret` and `Connection.UpdateSecretOk`.
- Removed the ability to unset a `Basic.Property` by invoking `del properties[key]`
- Removed the deprecated `pamqp.codec` sub-package

Add support for RabbitMQ versions < 3.6.0 by adding the pamqp.encode.support_deprecated_rabbitmq() method, restricting the data types that can be sent in a table-field value

- **Change Python versions supported to 2.7 and 3.4+**
- **Always decode field table keys as strings (6)**
- This may be a breaking change means in Python3 keys will always be type str for short strings. This includes frame
values and field table values.
- In Python 2.7 if a short-string (key, frame field value, etc) has UTF-8 characters in it, it will be a `unicode` object.
- Combine test coverage across all Python versions
- Fix range for signed short integer (7)
- Fix guards for usage of unsigned short usage in `pamqp.encode` (7)
- Fix encoding and decoding of unsigned short (7)
- Add support for unsigned short integer and long integer in field tables (10)
- Address edge case of small value in long type (8)
- Address long string encoding inconsistency (9)
- Cleanup unicode object & conditionals in py3 (9)
- Add `pamqp.exceptions.PAMQPException` as a base class for pamqp specific exceptions (4)
- Fix decoding of void values in a field table or array