Do not replace Optional with `| None` in ImplicitDict fields

[BenjaminPelletier]

Describe the bug
Currently, our autoformatters replace instances of Optional[ConcreteType] with ConcreteType | None. This results in a loss of clarity for expectations regarding ImplicitDict fields. For instance, this is more clear:

class Foo(ImplicitDict):
    baz: Optional[str]

...than

class Bar(ImplicitDict):
    baz: str | None

When a user is considering what to provide to populate the baz field, the Foo class type annotation reads as "this field may optionally contain a str" with the implication that it doesn't need to be provided since it's optional. The Bar class type annotation reads as "this field may be a str or it may be None" with the implication that a value with one of these types needs to be provided.

To reproduce

• Write an ImplicitDict class declare one or more fields with Optional[ConcreteType] type
• Run make format
• Observe that the fields have been rewritten to have the type ConcreteType | None

Difference from expected behavior
I expect to be able to annotate ImplicitDict fields as being optional using the word "Optional".