TST: fix intermittent dict_complex test failures

`test_dict_complex` and `test_dict_numpy_complex` would give occasional
test failures on Python 3.4.  The test was written to depend on the
order of values from `dict.values()`; remove that dependency.

Closes gh-7293.
Merge pull request #7298 from matthew-brett/fix-dict-complex-failures

TST: fix intermittent dict_complex test failures