Move pytest-runner back to setup_requires
Move pytest-runner back to setup_requires

pytest-runner needs to be defined in setup_requires,
because as it is a setuptools plugin, it must be
installed and available very early, and setup_requires
is the only place where that happens.

See https://github.com/pytest-dev/pytest/issues/3063

python-lz4/python-lz4,python-lz4/python-lz4