🏷️ Add mypy to the GitHub Action for tests and fixed types in the whole project (#655)

Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>

backend/app/api/deps.py

@@ -17,7 +17,7 @@ reusable_oauth2 = OAuth2PasswordBearer(
 )
 
 
-def get_db() -> Generator:
+def get_db() -> Generator[Session, None, None]:
     with Session(engine) as session:
         yield session