it seems to keep .tox envs even if one removes stuff from requirements file.
if that stuff causes troubles, as here with pytest-benchmark, the troubles stay...
pytest-benchmark (and its dependency "statistics") needs py 3.3+.
we do not want to run benchmarks on travis anyway, but even its presence
makes trouble due to this, so we just remove it from there again.
as --benchmark-skip is also removed from tox configuration, this means benchmarks
are run by default when pytest-benchmark is installed.
