tox.ini: Add {posargs} to py.test invocation
tox.ini: Add {posargs} to py.test invocation

This allows you to specify args on the tox command line that get passed through
to py.test -- e.g.:

tox -e py27 -- -k testResolve --tb=short setuptools/tests

pypa/setuptools,pypa/setuptools,pypa/setuptools