str.upper is not picklable under PyPy3

[ogrisel]

Report from the failing test ran with the PyPy3 environment configured by tox (not available on travis-ci):

__________________________________________________________________________________ CloudPickleTest.test_method_descriptors __________________________________________________________________________________

self = <tests.cloudpickle_test.CloudPickleTest testMethod=test_method_descriptors>

    def test_method_descriptors(self):
>       f = pickle_depickle(str.upper)

tests/cloudpickle_test.py:241: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
tests/cloudpickle_test.py:37: in pickle_depickle
    return pickle.loads(cloudpickle.dumps(obj))
cloudpickle/cloudpickle.py:605: in dumps
    cp.dump(obj)
cloudpickle/cloudpickle.py:107: in dump
    return Pickler.dump(self, obj)
../../opt/pypy3/lib-python/3/pickle.py:237: in dump
    self.save(obj)
../../opt/pypy3/lib-python/3/pickle.py:299: in save
    f(self, obj) # Call unbound method with explicit self
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <cloudpickle.cloudpickle.CloudPickler object at 0x0000000004568560>, obj = <function upper at 0x0000000001a32fc0>, name = 'upper'

    def save_function(self, obj, name=None):
        """ Registered with the dispatch to handle all function types.

            Determines what kind of function obj is (e.g. lambda, defined at
            interactive prompt, etc) and handles the pickling appropriately.
            """
        write = self.write

        if name is None:
            name = obj.__name__
        modname = pickle.whichmodule(obj, name)
        # print('which gives %s %s %s' % (modname, obj, name))
        try:
            themodule = sys.modules[modname]
        except KeyError:
            # eval'd items such as namedtuple give invalid items for their function __module__
            modname = '__main__'

        if modname == '__main__':
            themodule = None

        if themodule:
            self.modules.add(themodule)
            if getattr(themodule, name, None) is obj:
                return self.save_global(obj, name)

        # if func is lambda, def'ed at prompt, is in main, or is nested, then
        # we'll pickle the actual function object rather than simply saving a
        # reference (as is done in default pickler), via save_function_tuple.
>       if islambda(obj) or obj.__code__.co_filename == '<stdin>' or themodule is None:
E       AttributeError: 'builtin-code' object has no attribute 'co_filename'

[llllllllll]

fixed in #51