* 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.
Eliminated the use of "bare" TypeVars (i.e. a TypeVar that appears only once) within generic methods. While not considered an error in PEP 484, these are a common source of bugs in code, and some type checkers (including pytype and pyright) flag them as errors.
Co-authored-by: Eric Traut <erictr@microsoft.com>
