2to3 refactor broke vbenches

looks like lmap is now called by some vbenches, which breaks
the tests on all older versions.

.....................................................groupby_transform died with:
name 'lmap' is not defined
Skipping...

groupby_transform died with:
name 'lmap' is not defined
Skipping...

#4384, #4474

[jtratner]

@y-p how do the vbenches work? Are they supposed to contain only code that
works through the last X revisions?

test_perf is meant to allow before/after perf for sanity checking PRs, but the underlying vbench
tool and vbench suite is geared towards long term perf tracking.
With that in mind, vbenches should be portable as far back into pandas' history as is possible.