PyPy3-3.5-5.7.1-beta has issues in the traceback
library (https://travis-ci.org/gevent/gevent/jobs/219394025):

  During handling of the above exception, another exception occurred:

  Traceback (most recent call last):
    File "/home/travis/build/gevent/gevent/src/greentest/greentest.py", line 297, in wrap_restore_handle_error
      return method(self, *args, **kwargs)
    File "/home/travis/build/gevent/gevent/src/greentest/greentest.py", line 169, in wrap_timeout
      return method(self, *args, **kwargs)
    File "test__event.py", line 146, in test_set_exception
      self.assertRaises(greentest.ExpectedException, s1.get)
    File "/home/travis/.runtimes/versions/pypy3.5_571/lib-python/3/unittest/case.py", line 728, in assertRaises
      return context.handle('assertRaises', args, kwargs)
    File "/home/travis/.runtimes/versions/pypy3.5_571/lib-python/3/unittest/case.py", line 177, in handle
      callable_obj(*args, **kwargs)
    File "/home/travis/.runtimes/versions/pypy3.5_571/lib-python/3/unittest/case.py", line 201, in __exit__
      traceback.clear_frames(tb)
    File "/home/travis/.runtimes/versions/pypy3.5_571/lib-python/3/traceback.py", line 216, in clear_frames
      tb.tb_frame.clear()
  AttributeError: 'Frame' object has no attribute 'clear'

Fixes #958

[skip appveyor]