Simplify SerializerBases _convert_to_dict #166
Description
The SerializerBase class has a method _convert_to_dict which we rely on for serialization. It is currently using the old python inspection API using inspect.getfullargspec which should be replaced with the modern method inspect.signature.
This replacement is non-trivial due to inspect.getfullargspec being deeply ingrained in _convert_to_dict.
The _convert_to_dict method is in serious need of a refactoring anyways, so this replacement can happen when the method gets refactored.