[MNT] Add typecheck workflow using mypy

[MatthewMiddlehurst]

Closes #1856

Adds mypy as an optional workflow which can be run on PRs. The same can be run locally using mypy aeon/ after installing, as it uses the default set up currently.

I opted to avoid using pre-commit for this, as it is apparently prone to issues and should really be run on the whole repo instead of only changed files. (see https://jaredkhan.com/blog/mypy-pre-commit#step-2-creating-our-own-pre-commit-hook and python/mypy#13916)

[aeon-actions-bot]

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[baraline]

Didn't know about the tool but itdm definitly looks like the right one for the job. Test errors seems to be due to the hpy5 issue, so LGTM!