Use a custom install_command to work around tox-dev/tox#786.

pip-10-install.py

@@ -0,0 +1,19 @@
+"""
+Upgrade to pip 10 before installing requirements, working
+around error reported in tox-dev/tox#786
+"""
+
+import sys
+import subprocess
+import shlex
+
+
+def main():
+	subprocess.check_call(shlex.split(
+		'python -m pip install git+https://github.com/pypa/pip@release/10.0.0'
+	))
+	subprocess.check_call(shlex.split(
+		'python -m pip install') + sys.argv[1:])
+
+
+__name__ == '__main__' and main()

tox.ini

@@ -3,13 +3,11 @@ envlist = python
 minversion = 2.4
 
 [testenv]
+install_command = python pip-10-install.py {opts} {packages}
 deps =
 	setuptools>=31.0.1
 	# workaround for yaml/pyyaml#126
 	# git+https://github.com/yaml/pyyaml@master#egg=pyyaml;python_version=="3.7"
-
-	# test against PIP 10 beta
-	git+https://github.com/pypa/pip@release/10.0.0
 commands =
 	py.test {posargs}
 	python setup.py checkdocs