Add a wrapper for sign for NumPy-likes

[asmeurer]

Fixes #183

[asmeurer]

I'm getting this failure with dask locally:

________________________________________________________________________________ test_sign ________________________________________________________________________________

    @given(hh.arrays(dtype=hh.numeric_dtypes, shape=hh.shapes(), elements=finite_kw))
>   def test_sign(x):

array_api_tests/test_operators_and_elementwise_functions.py:1615:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

x = dask.array<reshape, shape=(), dtype=complex64, chunksize=(), chunktype=numpy.ndarray>

    @given(hh.arrays(dtype=hh.numeric_dtypes, shape=hh.shapes(), elements=finite_kw))
    def test_sign(x):
        out = xp.sign(x)
>       ph.assert_dtype("sign", in_dtype=x.dtype, out_dtype=out.dtype)
E       AssertionError: out.dtype=complex128, but should be complex64 [sign(complex64)]
E       Falsifying example: test_sign(
E           x=dask.array<reshape, shape=(), dtype=complex64, chunksize=(), chunktype=numpy.ndarray>,
E       )
E       Explanation:
E           These lines were always and only run by failing examples:
E               /Users/aaronmeurer/Documents/array-api-compat/array_api_compat/common/_aliases.py:537
E               /Users/aaronmeurer/Documents/array-api-compat/array_api_compat/common/_aliases.py:539
E               /Users/aaronmeurer/Documents/array-api-tests/array_api_tests/hypothesis_helpers.py:41
E               /Users/aaronmeurer/Documents/array-api-tests/array_api_tests/hypothesis_helpers.py:51
E               /Users/aaronmeurer/Documents/array-api-tests/array_api_tests/hypothesis_helpers.py:62
E               (and 28 more with settings.verbosity >= verbose)

array_api_tests/test_operators_and_elementwise_functions.py:1617: AssertionError

However, I cannot reproduce it:

>>> import array_api_compat.dask.array as xp
>>> x = xp.asarray(1+2j, dtype=xp.complex64)
>>> xp.sign(x)
dask.array<where, shape=(), dtype=complex64, chunksize=(), chunktype=numpy.ndarray>