class DataTable is detected as subclass of Callable[T]

[st-pasha]

19:29:19 /datatable_env/lib/python3.6/site-packages/py/_path/local.py:662: in pyimport
19:29:19     __import__(modname)
19:29:19 tests/__init__.py:14: in <module>
19:29:19     import datatable  # noqa
19:29:19 datatable/__init__.py:6: in <module>
19:29:19     from .fread import fread, FReader
19:29:19 datatable/fread.py:30: in <module>
19:29:19     **extra) -> DataTable:
19:29:19 /datatable_env/lib/python3.6/site-packages/typesentry/config.py:148: in prepared_decorator
19:29:19     sig = Signature(f, types, self)
19:29:19 /datatable_env/lib/python3.6/site-packages/typesentry/signature.py:52: in __init__
19:29:19     self._fill_from_inspection_spec(types)
19:29:19 /datatable_env/lib/python3.6/site-packages/typesentry/signature.py:132: in _fill_from_inspection_spec
19:29:19     self.retval.type = fann["return"]
19:29:19 /datatable_env/lib/python3.6/site-packages/typesentry/signature.py:364: in type
19:29:19     self._checker = checker_for_type(t)
19:29:19 /datatable_env/lib/python3.6/site-packages/typesentry/checks.py:57: in checker_for_type
19:29:19     checker = _create_checker_for_type(t)
19:29:19 /datatable_env/lib/python3.6/site-packages/typesentry/checks.py:106: in _create_checker_for_type
19:29:19     return MtCallable(t.__args__)
19:29:19 E   AttributeError: type object 'DataTable' has no attribute '__args__'