Tests failing with pypy

[jtojnar]

I tried running the tests on NixOS, Python 2 & 3 pass but pypy fails:

======================================================================
FAIL: test_clear (tests.test_orderedset.TestOrderedset)
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Traceback (most recent call last):
  File "/build/orderedset-2.0.1/tests/test_orderedset.py", line 89, in test_clear
    self.assertEqual(list(ws), [])
AssertionError: Lists differ: [frozenset([1])] != []

First list contains 1 additional elements.
First extra element 0:
frozenset([1])

- [frozenset([1])]
+ []

----------------------------------------------------------------------
Ran 32 tests in 0.031s

FAILED (failures=1)