* Add basic mypy workflow to CI
* Make the type check pass
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Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>
Co-authored-by: Nikita Sobolev <mail@sobolevn.me>
Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>
The `target-branch` field doesn't seem to support array.
Since it defaults to the default branch anyway, we should just remove the `target-branch` field from the config.