Drop support for old signature of IComposableState._on_setattr in 0.19

[jchodera]

YANK tests are emitting the following deprecation warning, which appears to be due to some changes required within openmmtools:

/home/travis/miniconda/envs/test/lib/python3.6/site-packages/openmmtools/states.py:2819: UserWarning: The signature IComposableState._on_setattr(self, standard_system, attribute_name) has been deprecated, and future versions of openmmtools will support only the new one: _on_setattr(self, standard_system, attribute_name, old_composable_state).
  'new one: {}.'.format(old_signature, new_signature))
/home/travis/miniconda/envs/test/lib/python3.6/site-packages/openmmtools/states.py:2819: UserWarning: The signature IComposableState._on_setattr(self, standard_system, attribute_name) has been deprecated, and future versions of openmmtools will support only the new one: _on_setattr(self, standard_system, attribute_name, old_composable_state).
  'new one: {}.'.format(old_signature, new_signature))
/home/travis/miniconda/envs/test/lib/python3.6/site-packages/openmmtools/states.py:2819: UserWarning: The signature IComposableState._on_setattr(self, standard_system, attribute_name) has been deprecated, and future versions of openmmtools will support only the new one: _on_setattr(self, standard_system, attribute_name, old_composable_state).
  'new one: {}.'.format(old_signature, new_signature))
/home/travis/miniconda/envs/test/lib/python3.6/site-packages/openmmtools/states.py:2819: UserWarning: The signature IComposableState._on_setattr(self, standard_system, attribute_name) has been deprecated, and future versions of openmmtools will support only the new one: _on_setattr(self, standard_system, attribute_name, old_composable_state).
  'new one: {}.'.format(old_signature, new_signature))
/home/travis/miniconda/envs/test/lib/python3.6/site-packages/openmmtools/states.py:2819: UserWarning: The signature IComposableState._on_setattr(self, standard_system, attribute_name) has been deprecated, and future versions of openmmtools will support only the new one: _on_setattr(self, standard_system, attribute_name, old_composable_state).
  'new one: {}.'.format(old_signature, new_signature))

[andrrizzi]

This is expected with the latest release. It's something that should be fixed in YANK. I'll edit the title to remind us to remove the deprecation warning and stop supporting the old signature in the next release.

[jchodera]

@andrrizzi : If this is a YANK issue and not an openmmtools issue, can you open an issue there and close this one?

[andrrizzi]

I've already fixed it there in choderalab/yank#1136. I'm keeping this open with the title changed because it's something we need to remember to do in the next release.