fix TypeError with use pyo3 0.2.7

[hhatto]

error occured with following code:

Traceback (most recent call last):
  File "usetokioloop.py", line 1, in <module>
    import tokio
  File "/path/to/tokio/__init__.py", line 14, in <module>
    class Loop(_tokio.TokioEventLoop, AbstractEventLoop):
TypeError: type 'tokio._tokio.TokioEventLoop' is not an acceptable base type

usetokioloop.py

import tokio

tokio.EventLoopPolicy()

Thanks