Remove the specification in tox.ini that _ is a builtin so that
it will no longer assume that _ does not need to be imported.
This helps ensure that the _ from i18n is used.
Change-Id: Ib701f92ff9d98dfcc655184caebce27ac4ab7bab
Allow the Zuul "Depends-On" to work better.
Adopting the solution developed by Doug Wiegley for LBaaS:
c61f920f2a54fd829ee37484b3743ab77b909847
Closes-Bug: #1492882
Change-Id: I7505a8b2ace94a0fe0b438e346011482932a339a