Well I sat on this for long enough that it got easier: async/await is no
longer a syntax error in any supported version of Python. That meant
that it was relatively straightforward to apply @nkonin's proposed patch
from #119.

Fixes #119.

All of the features from __future__ that were in this library have been
in Python since 3.0. Now that Python 2 is no longer supported, we no
longer need them.

The client module just broke 300 lines and had a lot of functionality in
it. This breaks the single file up into a package and reduces a decent
amount of duplication. Makes a few related changes:

- Replaces old Python __future__ imports with new ones
- Removes __all__ in favor of fewer imports
- Refactors common stream code into StreamClientBase
- Renames TCPPipeline to StreamPipeline for consistency (this is not a
  public class name)