util: restore the util.pickle symbol

This was accidently dropped in df56e6bd37f6, which started importing pickle
directly.  That commit explicitly says it will retain it for compatibility with
external stuff though.

The unused import in pycompat isn't flagged because that module is skipped.
Just importing with a comment seemed cleaner than `import X as Y` and then
assigning to a `pickle` variable, just to avoid the pyflakes warning.

Differential Revision: https://phab.mercurial-scm.org/D12419

mercurial/util.py

@@ -25,6 +25,7 b' import itertools'
 import locale
 import mmap
 import os
+import pickle  # provides util.pickle symbol
 import re as remod
 import shutil
 import stat

tests/test-check-pyflakes.t

@@ -24,4 +24,5 b' run pyflakes on all tracked files ending'
   mercurial/util.py:*:* undefined name 'file' (glob) (?)
   mercurial/encoding.py:*:* undefined name 'localstr' (glob) (?)
   tests/run-tests.py:*:* undefined name 'PermissionError' (glob) (?)
+  mercurial/util.py:*:* 'pickle' imported but unused (glob)