Pin to pip 19.0 for now for pypa/pip#6434.

pin-pip.py

@@ -0,0 +1,20 @@
+"""
+Downgrade to pip 19.0 before installing requirements, working
+around limitations introduced in 19.1 (ref
+https://github.com/pypa/pip/issues/6434)
+"""
+
+import sys
+import subprocess
+import shlex
+
+
+def main():
+	subprocess.check_call(shlex.split(
+		'python -m pip install pip<19.1'
+	))
+	subprocess.check_call(shlex.split(
+		'python -m pip install') + sys.argv[1:])
+
+
+__name__ == '__main__' and main()

tox.ini

@@ -5,6 +5,7 @@ minversion = 3.2
 tox_pip_extensions_ext_venv_update = true
 
 [testenv]
+install_command = python pin-pip.py {opts} {packages}
 deps =
 	setuptools>=31.0.1
 commands =