Test against 3 latest releases of mypy + hard pin in plugin hook na latest

[kornicameister]

]Ref: #43

• run CI with several mypy versions [ Test against several versions of mypy & loguru #59 ]
• pin the version (lowest) in plugin configuration [ Pin mypy version on lower boundary #60 ]
• update the documentation [ Add mypy compatibility note #61 ]

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[ThibaultLemaire]

This PR is a first PR to start tracking mypy version. No idea how low would you like to go, but if you have a suggestion to test even lower mypy version, let me know in the issue.

Good job on the PR! I was only thinking others might have higher expectations, personally I don't need anything below what I used for my PR (mypy==0.782) so it's already more than enough :)

If someone needs a lower version of mypy, they can always open an issue