Enable py::ellipsis on Python 2 (#2360)

* Enable py::ellipsis on Python 2

* Enable py::ellipsis tests on Python 2 and mention `Ellipsis` in the docs

tests/test_numpy_array.py

@@ -431,7 +431,6 @@ def test_array_create_and_resize(msg):
     assert(np.all(a == 42.))
 
 
-@pytest.unsupported_on_py2
 def test_index_using_ellipsis():
     a = m.index_using_ellipsis(np.zeros((5, 6, 7)))
     assert a.shape == (6,)