* Update version field in METADATA.toml.
* Import from collections.abc in Python 3 stub.
* Add six.__author__.
* Annotate PY2, PY3, and PY34 as literals.
* Correctly annotate *_type and replace instances where they were used
as type aliases.
* Use new union syntax in Python 2 stub.
* Use built-in generics in Python 2 stub where possible.
* Renamed _LazyDescriptor to _LazyDescr to match implementation.
* Derive _LazyDescr from object in Python 2 stub.
