Completely replace t.p.c.Interface with z.i.Interface

[ralphm]

twisted.python.components.Interface contains this snippet:

            # Don't warn for use within twisted, because twisted classes must
            # not be switched to z.i.Interface yet, b/c we forgot to deprecate
            # the "default" kwarg to __call__. Also when t.p.c.Interface is
            # removed, the rest of twisted can be updated simultaneously. But
            # everyone else should just use z.i.Interface directly now.

The deprecation will take place in Twisted 2.3, so after that we can completely switch to zope.interface.Interface.

[ralphm]

Grmbl. See #1636.