Add mypy-plugin extras to allow proper mypy version when using mypy plugin

[zhukovgreen]

Fixes https://github.com/dry-python/classes/issues/481

Can be merged after

• [ ] #487

I have made things!

Checklist

• [x] I have double checked that there are no unrelated changes in this pull request (old patches, accidental config files, etc)
• [ ] I have created at least one test case for the changes I have made
• [x] I have updated the documentation for the changes I have made
• [ ] I have added my changes to the CHANGELOG.md

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[eineruntervielen]

i encountered this problem last night. i adore your library. is their anything still open or why is this issue not merged yet? all the best, erik

[sobolevn]

Yes, we need to merge https://github.com/dry-python/classes/pull/487 first

[eineruntervielen]

thank you for you reply