py3: use pickle directly pycompat.pickle abstracted over the different pickle modules in Python 2 and 3. Now that we're Python 3 only, it is safe to use the `pickle` module directly. So this commit does that. As part of this we remove the rules from check-code.py that were forbidden direct pickle module use. We retain the `util.pickle` symbol for backwards compatibility, just in case some extensions were using it. Differential Revision: https://phab.mercurial-scm.org/D12249

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