[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

.gitlab-ci.yml

@@ -49,7 +49,7 @@ type:
   extends: .code_changes
   stage: checks
   script:
-    - mypy duniterpy tests examples
+    - mypy --install-types --non-interactive duniterpy tests examples
 
 lint:
   extends: .code_changes