Update tox for Ussuri, pin pip to 20.2.3 for tox

Tox dependencies are updated for Ussuri.

We need to pin the pip version, as at the time of this writing URL
dependencies are not supported. When they will be supported by a current
release this can be removed again.

See pypa/pip#8253

tox.ini

@@ -1,15 +1,18 @@
 [tox]
-envlist = py27,pep8
-minversion = 1.6
+envlist = py27,py36,pep8
+minversion = 3.2
 skipsdist = True
+requires = virtualenv >= 20
 
 [testenv]
+download = True
 usedevelop = True
-install_command = pip install -c {env:UPPER_CONSTRAINTS_FILE:https://raw.githubusercontent.com/sapcc/requirements/stable/queens-m3/upper-constraints.txt} -r requirements.txt -U {opts} {packages}
+install_command = pip install -c {env:UPPER_CONSTRAINTS_FILE:https://raw.githubusercontent.com/sapcc/requirements/stable/ussuri-m3/upper-constraints.txt} -r requirements.txt -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
+         VIRTUALENV_PIP=20.2.3
          PYTHONWARNINGS=default::DeprecationWarning
 deps = -r{toxinidir}/test-requirements.txt
-       -e{env:NEUTRON_SOURCE:git+https://github.com/sapcc/neutron.git@stable/queens-m3#egg=neutron}
+       -e{env:NEUTRON_SOURCE:git+https://github.com/sapcc/neutron.git@stable/ussuri-m3#egg=neutron}
 whitelist_externals = sh
 commands = python setup.py testr --slowest --testr-args='{posargs}'