Fix F402 violations and re-enable gating

F402 got stricter in hacking 0.9, so fix new violations and
re-enable gating.

Change-Id: I81a1b14311ea6dc7f01353aa27d1f18f1aec8e81
Fix the section name in CONTRIBUTING.rst

The name of the section "If you're developer, start here"
has changed in the wiki, now is "If you're a developer".
This commit updates it to correctly refer to the
proper section.

Change-Id: Ida0406aab1af92687b9e8087a328a5f0e8ce1d9a
Merge "Fix F402 violations and re-enable gating"