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