setup,py,requirements.txt: add pytest

setup,py,requirements.txt: add pytest

pytest requires pluggy >= 0.7, while we always use pluggy 0.6, as
specified by requirements.txt. as this version is good enough for
tox. but in tox.ini, we do use pytest, and no version is specified,
so we have good chance running into https://github.com/pytest-dev/pytest/issues/3753

also, remove pytest from tox.ini, as this dependency has been
added in requirements.txt

Signed-off-by: Kefu Chai <kchai@redhat.com>