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.

Author: bluetech

tox.ini

@@ -18,7 +18,9 @@ deps=
 commands=pytest {posargs:testing/benchmark.py}
 deps=
   pytest
-  pytest-benchmark
+  # Once pytest lifts its pluggy<1.0 constraint, the pytest-benchmark
+  # constraint can be removed.
+  pytest-benchmark==3.1.1
 
 [testenv:linting]
 skip_install = true