Travis CI: Add flake8 jobs for Python 2.7 and Python 3.7
An alternative approach to #21952 that runs flake8 in parallel Travis CI jobs on both Python 2.7 and Python 3.7. The Python 3.7 job is run in allow_failures mode.
See #21952 for more details on the --exclude and --select choices. A mix of these two PRs is also possible.
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines