Fix metaclass usage for ST3

This is a very nifty method to make metaclasses work for both ST2 and
ST3 since the Python 3 syntax for specifying metaclasses is incompatible
with Python 2 but instantiating the metaclass directly works fine. And
since baseclasses influence the used metaclass, this works just fine.

Really closes #29.

Source: https://github.com/ipython/ipython/blob/c0c14f8d36ccdfe2748dffa0a6269b6b01f8ea05/IPython/utils/py3compat.py#L242-L244
Use `%r` for setting debug instead of `"%s"`