recent change to `type.__dict__` incorrectly causes instances of classes with a metaclass to have readonly `__dict__` attribute in pyright

[DetachHead]

Code sample in pyright playground

class FooMetaclass(type): ...

class Foo(metaclass=FooMetaclass): ...

# Cannot access attribute "update" for class "MappingProxyType[str, Any]"
#   Attribute "update" is unknown (reportAttributeAccessIssue)
Foo().__dict__.update(a=1)

this works at runtime, and was not a type error prior to pyright 1.1.408 (which introduced #14951)

[srittau]

I'm not sure why pyright assumes that Foo().__dict__ is a MappingProxyType in your example. At runtime it is a dict, and mypy seems to agree.