Pin pip to 20.2.3 for tox

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

@@ -1,17 +1,22 @@
 [tox]
 envlist = py27,py36
-minversion = 1.6
+minversion = 3.2
 skipsdist = True
+requires = virtualenv >= 20
 
 [testenv]
 usedevelop = True
 install_command = pip install -c {env:UPPER_CONSTRAINTS_FILE:https://raw.githubusercontent.com/sapcc/requirements/stable/ussuri-m3/upper-constraints.txt} -r requirements.txt -r test-requirements.txt -U {opts} {packages}
+# pip 20.2.3 is used until constraints as urls are reintroduced in pip
+# see https://github.com/pypa/pip/issues/8253 or PR 9673 (currently merged, not released)
 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/ussuri-m3#egg=neutron}
 whitelist_externals = sh
 commands = python setup.py testr --slowest --testr-args='{posargs}'
+download = True
 
 [testenv:pep8]
 commands =