typehints.TypeVariable issues with __hash__

This class currently does not define a __hash__ function. This defies the assumption made by sets and dicts which contain TypeConstraint objects.

In Python 3, classes which define ___eq___ also need to explicitly define ___hash___ to be hashable. However, this causes _combiners_test.CombineTest.test_to_list_and_to_dict_ to throw an error in slow_stream.py (adding NoneType and long).

There are multiple TypeVariable instances with name 'K' or 'V' throughout the codebase. The equality of these instances may possibly cause issues.

Imported from Jira [BEAM-3730](https://issues.apache.org/jira/browse/BEAM-3730). Original Jira may contain additional context.
Reported by: luke-zhu.

Start with the TypeVariable and TypeConstraint definitions and reproduce _combiners_test.CombineTest.test_to_list_and_to_dict_, which currently fails in slow_stream.py when adding NoneType and long. Trace the equality and hashing behavior of TypeVariable instances, including repeated names such as K and V. Done means the relevant collections remain usable and the named test passes without introducing incorrect equality between distinct variables.

Written by the indexing model from the issue text.