fixes Python3.5 "yield in async function" errors

[ph1l]

fixes #4 but breaks tests.

I don't quite understand all this async stuff, or how to fix this test:

=========================================== ERRORS ============================================
_____________________ ERROR collecting secret_handshake/test_boxstream.py _____________________
.eggs/pytest-3.1.3-py3.5.egg/_pytest/python.py:408: in _importtestmodule
    mod = self.fspath.pyimport(ensuresyspath=importmode)
.eggs/py-1.4.34-py3.5.egg/py/_path/local.py:662: in pyimport
    __import__(modname)
E     File "/home/elektron/code/PySecretHandshake/secret_handshake/test_boxstream.py", line 74
E       assert [msg async for msg in unbox_stream] == [b'foo', b'foo', b'bar']
E                       ^
E   SyntaxError: invalid syntax
!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!
=================================== 1 error in 0.20 seconds ===================================

[coveralls]

Coverage decreased (-0.5%) to 79.521% when pulling f28f6c9 on ph1l:fix_yield_inside_async_func into e70324e on pferreir:master.