Pamqp

This release fixes an issue with `Basic.Reject` `requeue=False` always being set to `True` (29 - [eandersson](https://github.com/eandersson))

This represents the first stable release of the 3.0 branch for pamqp. It is Python 3.6+ only and is focused on protocol correctness and completeness.

Changes
- Bugfix for encoding of unsigned small integers being treated as signed small integers
- Updated tests around timezone behavior issues

- Rename `pamqp.frame._frame_parts` to `pamqp.frame.frame_parts` (15 again)
- `pamqp.commands.Basic.QoS.global_` renamed to `pamqp.commands.Basic.QoS.globally`
- Removed mypy checking due to errors in mypy and recursive type aliases
- Added `pamqp/py.typed` for PEP-561 compatibility (21 - [michael-k](https://github.com/michael-k))

- Refactor codegen.py
- Revert the behaviors added in 3.0.0a2 with regard to documented defaults and `None`
- Use `amqp0-9-1.extended.xml` instead of `amqp-0-9-1.xml` to get the documentation for RabbitMQ added classes/methods
- Add strict value checking for deprecated values
- Remove empty ``__init__`` functions from method classes

- Make comparison of Basic.Properties against other object types raise `NotImplementedError`
- Return test coverage to 100%

- Added mypy as part of the test pipeline and made updates based upon its findings.
- Added length checking and regex checking for values specified in AMQP spec
- Fixed some of the type annotations added in 3.0.0a0
- Fixed some of the documentation and label usage in `pamqp.commands`
- Removed redundant inline documentation in `pamqp.commands`
- Updated default values to only reflect defaults specified in the XML and JSON specs. If no default is specified, the value will now be `None`.