Update warning filter for SetupTools and filelock

SetupTools 58.3.0 deprecated use of the setup_requires parameter as
well as calls to easy_install and setup.py install. The filelock
library (used by virtualenv, on which tox relies) is replacing its
poll_intervall parameter with poll_interval and has deprecated the
former in a 3.4.0 release. Ignore these with the PYTHONWARNINGS in
tox.ini so they'll be filtered out.