pyflakes: use pycompat.pickles to prevent error

The pyflakes in my test box complain about pickle in pycompat.

  mercurial/pycompat.py:17: 'pickle' imported but unused

mercurial/pycompat.py

@@ -15,6 +15,7 b' try:'
     pickle.dumps
 except ImportError:
     import pickle
+    pickle.dumps # silence pyflakes
 
 try:
     import cStringIO as io