[ci] Install mypy attrs stubs to check types classes

https://mypy-lang.blogspot.com/2021/06/mypy-0900-released.html
https://mypy-lang.blogspot.com/2021/06/mypy-0910-released.html

Mypy v0.900 no longer install additional stubs by default
Third-party Library Stubs in Stub Packages (Breaking Change)

attrs for duniterpy.documents.ws2p.heads

https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports