Avoid storing empty singleton on class itself in python implementation

[apmorton]

import dill
import frozendict


def test_foo():
    # this will work
    dill.dumps(frozendict.frozendict(a=1))

    # construct the first empty frozendict
    frozendict.frozendict()

    # this will blow up with:
    # dill._dill.PicklingWarning: Cannot locate reference to <class 'frozendict.core.frozendict'>.
    dill.dumps(frozendict.frozendict(a=1))

There are a few things going on here, but this solves the immediate problem that anyone anywhere constructing an empty frozendict when using the python implementation will change the behavior for subsequent calls to dill.dumps.

The larger issue, that I don't really know how to solve satisfactorily, is that dill thinks it needs to pickle the class itself rather than a reference to it. I will open an issue with more details about that.

See: #87 #88