Except in:
- docs/writing-bots-guide.md, because bots are supposed to be Python 2
compatible
- puppet/zulip_ops/files/zulip-ec2-configure-interfaces, because this
script is still on python2.7
- tools/lint
- tools/linter_lib
- tools/lister.py
For the latter two, because they might be yanked away to a separate repo
for general use with other FLOSS projects.
For performance reasons, we spawn each linter in a separate OS thread.
The downside of this is that all lints would end up in stdout without
much visual separation, resulting in confusing error log. This commit
introduce the `print_err` function, which shows which linter each line
of lint is from.