Limit flake8 to a single job per test

[timj]

Without this change flake8 creates many subprocesses every time
it is invoked. This causes significant overhead and slows down
test execution. In conjunction with pytest-xdist many hundreds
of additional flake8 processes can be created which causes
significant load issues.

[timj]

I am getting the tox failure even on master so I'm not sure what is going wrong. This patch fixes my local installation and makes things significantly faster.

[peleccom]

if case this is true
"Without this change flake8 creates many subprocesses every time
it is invoked. "
it should be fixed.
For some CI servers (CirlceCI for example) it might cause serious performance issues. It is 22 cpus cores visible but due to virtualization design only 2 can be used on default free configuration. flake8 should be single thread on CI servers

I am looking forward for this change merged.

In connection to failed Travis build. I think it was temporary issues on Travis or Github. I think new build could fix this error.

[timj]

I've rebased against current master and I hope the travis checks work this time. This fix really makes a huge performance difference when using this plugin and is critical for people who use pytest-xdist.

[timj]

@tholo I rebased and I still get tox errors. They don't seem to be related to my PR.

[timj]

Rebasing again has finally sorted out the travis failures.

[timj]

Is there anything else I can do to help this PR get merged?

[frossie]

@tholo What can we do to help this PR get merged?

[timj]

It looks like flake8 has been modified to make this issue go away.