Fix NestedField Type hint for Lambda Schema types

[somethingnew2-0]

Providing a lambda with a returning callable type is handled by Marshmallow (such as below), but not allowed by the type-hinting. Fix that

class PrimarySchema(Schema):
    other = fields.Nested(lambda: OtherSchema)

[lafrech]

Thanks.

Would you like to add yourself to AUTHORS file?

I'm wondering if we could be more specific than type. Could we enforce that it is just not any type but only schema types? Like, by specifying SchemaMeta rather than type? This question applies to the whole codebase, so we may merge anyway, but I'd be interested to gather thoughts about this.

[somethingnew2-0]

@lafrech added myself to AUTHORS

SchemaMeta does appear to work in my codebase to be more specific than type. But I agree that it should probably be updated everywhere in marshmallow where type was used such as on line 536 of the same method https://github.com/marshmallow-code/marshmallow/pull/2164/files#diff-0cddc4aac0e4e16946830a5807c264d65b7f14a2a1b1cb23b03028a36df3681aR536

[lafrech]

Yes, exactly. Thanks for the feedback. This can be done in another (welcome) PR.