* CI: add a workflow to show flake8 violations in modified files of a PR
* modify a file to trigger the lint check
* CI: add a workflow to show mypy violations in modified files of a PR
* modify a file to trigger the type check
* Split flake8 and mypy into two parallel jobs; run a variant of the workflow on push event; modify a file to trigger the push workflow
* fail the task if there are syntax errors; remove old lint workflow
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