Temporarily pin tox<=1.6.1

The release of tox 1.7.0 happened today, and we're clearly not ready
for it yet. Stop the bleeding by sticking with 1.6.1 while we try to
sort out the issues.

Change-Id: I3289828ea9a0edadcf40c06212b0925957b99f73
Related-Bug: #1274135