Align local pre-commit with CI-based linting (Tox and Travis)

[benjaoming]

Summary

• Removes frontendlint and pythonblack tox entries (they are now part of "pythonlint3")
• ...also removes said entries from Travis, hence quicker
• All calls to pre-commit now happen through the same configuration
• Caching on travis potentially improved

Does anyone else get frustrated when their code breaks a CI linting check because black isn't in pre-commit?

Having it in pre-commit is much faster, as it just lints changed files.

Reviewer guidance

I didn't care to pin the black version. Does it matter?

Edit: We use Black without pinning in tox.ini as well, so we always run the latest version.

References

This partly solves #5409

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Contributor Checklist

PR process:

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Testing:

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

Reviewer Checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[codecov]

Codecov Report

Merging #6583 into develop will increase coverage by 0.00%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
kolibri/core/content/utils/check_schema_db.py	100.00% <0.00%> (+8.69%)	⬆️

[benjaoming]

This is waiting for the other PRs to be merged, only then can we get the green lights from Travis ☁️ 🚥

[indirectlylit]

I'm a bit confused how it "knows" to use a py3 venv for black - e.g. where would this be specified?

However I tested with a py2.7 environment and confirmed that it seems to work

[benjaoming]

I'm a bit confused how it "knows" to use a py3 venv for black - e.g. where would this be specified?

I think it's in the hooks configuration:

https://github.com/psf/black/blob/master/.pre-commit-hooks.yaml