TYP: enable reportPropertyTypeMismatch
This might be controversial. The setter allows any `Mapping` but the getter returns a `dict`.

[reportPropertyTypeMismatch](https://github.com/microsoft/pyright/blob/main/docs/configuration.md):

> [...] type of the value passed to the setter is not assignable to the value returned by the getter. Such mismatches violate the intended use of properties, which are meant to act like variables. The default value for this setting is 'error'.