Pin pip<19.1 in tox

Fixes ansible#2006

Ref pypa/pip#6434
Ref jaraco/skeleton@123b0b2
Ref tox-dev/tox#1276

tox.ini

@@ -11,6 +11,11 @@ skip_missing_interpreters = True
 isolated_build = True
 
 [testenv]
+# Hotfix for https://github.com/pypa/pip/issues/6434
+# Based on https://github.com/jaraco/skeleton/commit/123b0b2
+# Check https://github.com/tox-dev/tox/issues/1276 for the final solution
+install_command =
+    python -c 'import subprocess, sys; pip_inst_cmd = sys.executable, "-m", "pip", "install"; subprocess.check_call(pip_inst_cmd + ("pip<19.1", )); subprocess.check_call(pip_inst_cmd + tuple(sys.argv[1:]))' {opts} {packages}
 usedevelop = True
 passenv = *
 setenv =