Pypy refcheck
``np.resize(a, refcheck=True)`` relies on refcount semantics to check that no other object holds a reference to ``a``.  Since PyPy only partially mocks refcount semantics, the check is unreliable on PyPy. Unfortunately ``a`` (or rather ``uniques`` in this case) is allocated all the way out at user space, so it might be problematic to always use ``refcheck=False``. Instead I percolated ``refcheck`` as a kwarg with default value ``True`` for safety out to the point at which ``uniques`` is allocated. See also issue #15854.

With this change, on PyPy, failing tests that use hashtables now pass
