Cap maximal cyclomatic complexity

flake8 has support for cyclomatic complexity (Mccabe) currently our
worst offender has a complexity of 28. So
set our max-complexity to 29 so that don't make things any worse.

https://github.com/flintwork/mccabe

Change-Id: Ib6db9de92b3e6e196008580318a2139976610349