mypy workflow setup

[to-sta]

Contributor checklist

• This pull request is on a separate branch and not the main branch

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Description

In this PR i have added mypy (Static Type Checking for python) to the backend workflow. This is the last missing piece for the issue #329.

I tested the cli command locally and then run the workflow via act.

Comments:
While looking into the workflows, i was thinking that we should rename the files now. pr_backend_formatting_check does linting and static type checking. I think pr_ci_backend would make sense. I will open an issue for that and a few more minor things regarding workflows.

Related issue

• Look into isort and mypy for backend environment #329

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[andrewtavis]

Thanks for the edits here @to-sta as well as the new issues to keep the setup improvements rolling! 🚚😊