The latest version of flake8 requires Python 3.4
The latest version of flake8 requires Python 3.4

Since 3.4 didn't really introduce new syntax, we can use that for
linting with flake8 as well. Also allows us to use xenial instead of
trusty.

SublimeText/PackageDev,SublimeText/AAAPackageDev,SublimeText/AAAPackageDev