Design Py3-compatible typehints annotation support in Beam 3.

Existing [Typehints implementaiton in Beam|[https://github.com/apache/beam/blob/master/sdks/python/apache_beam/typehints/
] heavily relies on internal details of CPython implementation, and some of the assumptions of this implementation broke as of Python 3.6, see for example: https://issues.apache.org/jira/browse/BEAM-6877, which makes typehints support unusable on Python 3.6 as of now. Python 3 Kanban Board lists several specific typehints-related breakages, prefixed with "TypeHints Py3 Error".

We need to decide whether to:

• Deprecate in-house typehints implementation.
• Continue to support in-house implementation, which at this point is a stale code and has other known issues.
• Attempt to use some off-the-shelf libraries for supporting type-annotations, like Pytype, Mypy, PyAnnotate.

WRT to this decision we also need to plan on immediate next steps to unblock adoption of Beam for Python 3.6+ users. One potential option may be to have Beam SDK ignore any typehint annotations on Py 3.6+.

cc: udim, altay, robertwb.