Avoid pip breakage due to keyring

Apparently under linux pip can become fully blocked due to keyring
presence. That is a known open bug, so we apply the workaround until
it is fixed.

Change-Id: I30a5ec1b04b57a5604cb3caa3bc56ea2476e89ba

tox.ini

@@ -8,6 +8,10 @@ usedevelop = True
 install_command = pip install -U {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
+   # keyring presence can fully block pip on linux
+   # https://github.com/pypa/pip/pull/8687
+   # https://github.com/pypa/pip/issues/6773#issuecomment-515857335
+   PYTHON_KEYRING_BACKEND=keyring.backends.null.Keyring
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = python setup.py testr --slowest --testr-args='{posargs} tests.unit'