Re-define TypeCodecBase as an AbstractBaseClass

We should subclass TypeCodecBase from abc.ABC because:

• it makes the API clear and unambiguous
• it ensures that the user implements all methods that are needed by the custom type marshaling machinery (classes with abc.abstractmethod s and abc.abstractproperty s that have not been overridden raise an exception upon initialization)
• it allows us to validate certain aspects of the user-implemented type codecs that are otherwise difficult to introspect and/or validate

In order to implement this, we will need to split TypeCodecBase into 2 (or possibly 3) abstract base classes:

• a class to inherit from when defining a type codec that only coverts a custom python type to something BSON understands
• a class to inherit from when defining a type codec that only converts a certain BSON type into a custom python type
• a class to inherit from when defining a codec that implements a 2-way conversion.  This can also be achieved by simply inheriting from both of the base classes described above so a decision to introduce it would be based primarily on considerations of user-experience