Test fail on Python 3.7

[hroncok]

[terminado (master)]$ git rev-parse HEAD
7fde385926d36ddc5d5475cee275e41e02bce505
[terminado (master)]$ python3.7 -m venv __venv__
[terminado (master)]$ . __venv__/bin/activate
(__venv__) [terminado (master)]$ pip install tornado ptyprocess pytest
...
(__venv__) [terminado (master)]$ python -m pytest
============================= test session starts ==============================
platform linux -- Python 3.7.0rc1, pytest-3.6.2, py-1.5.3, pluggy-0.6.0
rootdir: /home/churchyard/Dokumenty/RedHat/terminado, inifile:
collected 8 items                                                              

terminado/tests/basic_test.py ......F.                                   [100%]

=================================== FAILURES ===================================
______________________ UniqueTermTests.test_max_terminals ______________________

self = <terminado.tests.basic_test.UniqueTermTests testMethod=test_max_terminals>

    @tornado.testing.gen_test
    def test_max_terminals(self):
        tms = yield self.get_term_clients(['/unique'] * MAX_TERMS)
>       pids = yield self.get_pids(tms)

terminado/tests/basic_test.py:228: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
__venv__/lib64/python3.7/site-packages/tornado/gen.py:1099: in run
    value = future.result()
__venv__/lib64/python3.7/site-packages/tornado/gen.py:1107: in run
    yielded = self.gen.throw(*exc_info)
terminado/tests/basic_test.py:121: in get_pids
    pid = yield tm.get_pid()
__venv__/lib64/python3.7/site-packages/tornado/gen.py:1099: in run
    value = future.result()
__venv__/lib64/python3.7/site-packages/tornado/gen.py:1107: in run
    yielded = self.gen.throw(*exc_info)
terminado/tests/basic_test.py:87: in get_pid
    (stdout, extra) = yield self.read_stdout()
__venv__/lib64/python3.7/site-packages/tornado/gen.py:1099: in run
    value = future.result()
__venv__/lib64/python3.7/site-packages/tornado/gen.py:1113: in run
    yielded = self.gen.send(value)
terminado/tests/basic_test.py:74: in read_stdout
    stdout = "".join([msg[1] for msg in msglist if msg[0] == 'stdout'])
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

.0 = <list_iterator object at 0x7f68fea54128>

>   stdout = "".join([msg[1] for msg in msglist if msg[0] == 'stdout'])
E   TypeError: 'NoneType' object is not subscriptable

terminado/tests/basic_test.py:74: TypeError
------------------------------ Captured log call -------------------------------
autoreload.py              123 WARNING  tornado.autoreload started more than once in the same process
=============================== warnings summary ===============================
terminado/tests/basic_test.py::TestTermClient
  cannot collect test class 'TestTermClient' because it has a __init__ constructor

-- Docs: http://doc.pytest.org/en/latest/warnings.html
=============== 1 failed, 7 passed, 1 warnings in 40.72 seconds ================

Funny enough a second run of tests succeeded:

$ python -m pytest
============================= test session starts ==============================
platform linux -- Python 3.7.0rc1, pytest-3.6.2, py-1.5.3, pluggy-0.6.0
rootdir: /home/churchyard/Dokumenty/RedHat/terminado, inifile:
collected 8 items                                                              

terminado/tests/basic_test.py ........                                   [100%]

=============================== warnings summary ===============================
terminado/tests/basic_test.py::TestTermClient
  cannot collect test class 'TestTermClient' because it has a __init__ constructor

-- Docs: http://doc.pytest.org/en/latest/warnings.html
==================== 8 passed, 1 warnings in 40.39 seconds =====================

And I needed to remove all the pyc files and recreate the venv to reproduce the failure.

[encukou]

Probably related to #21
I reproduced this when running the tests with pytest (as Terminado's Travis config does), but I could not reproduce when I used Nose (as Terminado's README instructs).