tox.ini: Add flake8 tests to find Python syntax errors and undefined names

[cclauss]

This change is 

[k8s-ci-robot]

Hi @cclauss. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[Ark-kun]

/lgtm

[cclauss]

Is something more required?

[Ark-kun]

/cc @hongye-sun

[k8s-ci-robot]

@Ark-kun: GitHub didn't allow me to request PR reviews from the following users: hon.

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[hongye-sun]

/ok-to-test
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hongye-sun

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[Ark-kun]

/test kubeflow-pipeline-e2e-test