Fix %bookmark -l for Python 3

Closes gh-5676

Also add a simple test for %bookmark

IPython/core/magics/osm.py

@@ -663,8 +663,7 b' class OSMagics(Magics):'
         elif 'r' in opts:
             bkms = {}
         elif 'l' in opts:
-            bks = bkms.keys()
-            bks.sort()
+            bks = sorted(bkms)
             if bks:
                 size = max(map(len, bks))
             else:

IPython/core/tests/test_magic.py

@@ -942,3 +942,10 b' def test_edit_cell():'
     
     # test
     _run_edit_test("1", exp_contents=ip.user_ns['In'][1], exp_is_temp=True)
+
+def test_bookmark():
+    ip = get_ipython()
+    ip.run_line_magic('bookmark', 'bmname')
+    with tt.AssertPrints('bmname'):
+        ip.run_line_magic('bookmark', '-l')
+    ip.run_line_magic('bookmark', '-d bmname')