Add environment variable {toxinidir} in tox.ini
Change-Id: Iee0e6e1f5ea49a7307bdfd493b4203df17369d75
install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
[testenv:releasenotes]
-deps = -rtest-requirements.txt
+deps = -r{toxinidir}/test-requirements.txt
commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html