gh-40880: update mypy config
    
as our minimal python is now 3.11

### 📝 Checklist

- [x] The title is concise and informative.
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URL: #40880
Reported by: Frédéric Chapoton
Reviewer(s):

Author: Release Manager

src/mypy.ini

@@ -1,6 +1,6 @@
 # https://mypy.readthedocs.io/en/stable/config_file.html
 [mypy]
-python_version = 3.9
+python_version = 3.11
 
 # https://github.com/sagemath/sage/issues/36198#issuecomment-1709521850
 follow_imports = skip