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)];
+    });
 }