Integrate Zulint into Zulip Terminal.

[amanagr]

Zulip Terminal uses linters like isort, mypy, pep8 and pytest for running tests. Integrate all into Zulint and add it to Zulip Terminal.

[preetmishra]

@amanagr Great! I was going to bring this up in #zulip-terminal, my idea was to add a section in the README where we would explicitly encourage contributors to run isort, mypy, pycodestyle along with pytest locally before pushing their changes but Zulint seems more plausible. This will greatly help new contributors to make better commits.

[neiljp]

@amanagr I favour bringing zulint into the project, but at least initially my preference would be to use it for additional linting.

We could then discuss integrating it with the others later. I enjoy seeing different linting pass/fail in CI, and if zulint supports splitting linting elements up I feel that integration could be useful, though I'm not sure if including pytest fits.

This is related to #508 (meta-linters)

@preetmishra There is some text there regarding this, but if you have ideas of how to improve it, please discuss or propose a change :)

[preetmishra]

The text in the README is pretty descriptive already though I think it might be useful to add a redirect link to the README (or maybe add a section) in the message that @zulipbot comments whenever someone claims any issue for the first time.

[zulipbot]

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[neiljp]

This is being most recently worked on in #980.