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
diff --git a/tests/test_numpy_array.cpp b/tests/test_numpy_array.cpp
index 156a3bf..e37beb5 100644
--- a/tests/test_numpy_array.cpp
+++ b/tests/test_numpy_array.cpp
@@ -382,9 +382,7 @@
         return a;
     });
 
-#if PY_MAJOR_VERSION >= 3
-        sm.def("index_using_ellipsis", [](py::array a) {
-            return a[py::make_tuple(0, py::ellipsis(), 0)];
-        });
-#endif
+    sm.def("index_using_ellipsis", [](py::array a) {
+        return a[py::make_tuple(0, py::ellipsis(), 0)];
+    });
 }
diff --git a/tests/test_numpy_array.py b/tests/test_numpy_array.py
index 2c977cd..1b6599d 100644
--- a/tests/test_numpy_array.py
+++ b/tests/test_numpy_array.py
@@ -431,7 +431,6 @@
     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,)