Adjusted AbjadObject __metaclass__ syntax.

The class AbjadObject is now created with its abc.ABCMeta metaclass via a
syntax compatible with both Python 2.7.x and 3.x.

Note that the user-friendly metaclass syntax found in Python 2 and Python 3 is
completely incompatible.

In 2.x:

    class MyClass(object):
        __metaclass__ = abc.ABCMeta

In 3.x:

    class MyClass(metaclass=abc.ABCMeta):
        pass

Python 3 does not recognize the __metaclass__ class variable, while Python 2
considers the metaclass keyword to be a syntax error.

However, we can still create cross-version compatible abstract classes via
dynamic class creation:

    AbstractBase = abc.ABCMeta(
        'AbstractBase', (), {'__metaclass__': abc.ABCMeta},
        )

    class MyClass(AbstractBase):
        pass

With the above construct, both Python 2 and Python 3 will recognize MyClass as
having ABCMeta as its metaclass.

AbjadObject now follows the above creation pattern. All tests pass, and all
docs build as before.