mutable-override does not understand unions

Bug Report
If I enable mutable-override it fails as it does not understand simple unions.

To Reproduce
Turn on mutable-override

class Base:
    val: bool|None = True
    
class Derived(Base):
    val = False

Gist: https://gist.github.com/mypy-play/aeec67e97957ea9e60431ce8466d8af3
Playground: https://mypy-play.net/?mypy=latest&python=3.12&enable-error-code=mutable-override&gist=aeec67e97957ea9e60431ce8466d8af3

Expected Behavior
no error

Actual Behavior

main.py:5: error: Covariant override of a mutable attribute (base class "Base" defined the type as "bool | None", expression has type "bool")  [mutable-override]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.18.2
• Mypy command-line flags: enable-error-code="mutable-override"
• Python version used: 3.12

Riproduci il report con l’esempio Base/Derived fornito su mypy 1.18.2 con mutable-override abilitato, quindi traccia il punto di ingresso del controllo dei tipi di mutable-override per le annotazioni union. Il lavoro è completato quando l’esempio non produce errori, mentre i controlli esistenti sull’override degli attributi mutabili continuano a funzionare.