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.
Alternately, we can skip the next patch and just restore pickle = pycompat.pickle. But since there were check-code rules pointing toward util.pickle, I initially thought to cleanup pycompat a bit. That is at odds with the end of the series, which keeps several unused imports in pycompat.