Python: Extend the use of the singleton

[Fokko]

Instead of having a specialized caching mechanism per class, we can generalize it as a singleton metaclass that we can reuse across different classes.

This moves the Singleton out of the types.py and implements it as a singleton.

Changing Class(ABC) to Class(metaclass=ABCMeta) is equivalent, but nicer since it more explicitly shows that we're setting a metaclass. From abc:

class ABC(metaclass=ABCMeta):
    """Helper class that provides a standard way to create an ABC using
    inheritance.
    """
    __slots__ = ()

[rdblue]

Why do we want the reader to know that this is changing a metaclass and not just inheriting from ABC? It seems like bringing a metaclass into the code is more clear about what's happening, but introduces a distinction that people (like me) may not understand or need.

[rdblue]

Looks fine to me. It's nice that it removes more code.