We've already dropped py2 support and switched to py3. This was missed.
Change-Id: I8484ffc8993b28fca25885d3f712ac5c0a8939b8
Partially-Implements: blueprint drop-py2-support
bindep installs a slew of stuff we don't really need and doesn't add
much if any value. This only turns on bindeps that are required.
This shaves about 3 minutes off each gate run.
Closes-Bug: #1620865
Change-Id: I24df4f2a30c682c1b09e89c7c52f7608582da1a6