Pin an older pytest-benchmark for now
Pin an older pytest-benchmark for now

Problem: when tox asks to install pytest, it gets pytest 3.2.5, I guess
because later versions have `pluggy<1.0` and pluggy's version is now
1.0.0.dev0. Which happens to work fine for the tests, but not for the
benchmark, since pytest-benchmark requires `pytest>=3.8`.

Solution: pin to an older pytest-benchmark for now.

pytest-dev/pluggy,hpk42/pluggy,RonnyPfannschmidt/pluggy,pytest-dev/pluggy,RonnyPfannschmidt/pluggy