Type signature for AvroSerializer.__init__()'s to_dict parameter is incorrect

[jonbuffington]

Description

In release 2.7.0, the type signature for AvroSerializer.__init__()'s to_dict parameter is incorrect. The type signature does not allow None as a value but the implementation defaults to and supports None as a value.

The type signature for the to_dict parameter should be modified to support being optional (example for Python 3.12):

to_dict: Callable[[object, SerializationContext], dict] | None = None,

How to reproduce

Use pyright to check a module that calls AvroSerializer(…):

…
avro_deserializer = AvroDeserializer(schema_registry_client)
…

and you will get the following error:

error: Argument of type "None" cannot be assigned to parameter "from_dict" of type "(dict[Unknown, Unknown], SerializationContext) -> object" in function "__init__"
    Type "None" is not assignable to type "(dict[Unknown, Unknown], SerializationContext) -> object" (reportArgumentType)

[jonbuffington]

The incorrect type signature also applies to AvroDeserializer.