Add test for complex numpy arithmetic #1433.

tests/run/complex_extern_GH1433.pyx

+# tag: numpy
+
+cimport numpy as np
+
+def divide(np.float64_t x, np.complex128_t y):
+    """
+    >>> divide(2, 1+1j)
+    (1-1j)
+    """
+    return x / y
+
+def pow(np.complex128_t x, np.complex128_t y):
+    """
+    >>> pow(1 + 1j, 2j)  # doctest: +ELLIPSIS
+    (0.1599...+0.1328...j)
+    """
+    return x ** y