stash tests fail on Windows

[gsnedders]

The tests from #14024 fail on Windows because multiprocessing on Windows uses its "spawn" start method which relies on being able to pickle everything (as there is no fork on Windows), and they currently can't be pickled.

Found in #16936, filing mostly to have something to link to from xfail definitions.

e.g.,

______________________________ test_delayed_dict ______________________________

add_cleanup = <function add at 0x0000000005E61B38>

    def test_delayed_dict(add_cleanup):
        """Ensure that delays in proxied `dict` retrieval do not interfere with
        initialization in parallel threads."""
    
        class SlowDict(BaseManager):
            pass
    
        request_lock = multiprocessing.Lock()
        response_lock = multiprocessing.Lock()
    
        queue = multiprocessing.Queue()
    
        # This request handler allows the caller to delay execution of a thread
        # which has requested a proxied representation of the "get_dict" property.
        def mutex_dict_request():
            """This request handler allows the caller to delay execution of a
            thread which has requested a proxied representation of the `get_dict`
            property, simulating a "slow" interprocess communication channel."""
            request_lock.release()
            response_lock.acquire()
            return {}
    
        SlowDict.register("get_dict", callable=mutex_dict_request)
        SlowDict.register("Lock", callable=lambda: threading.Lock())
    
        slowdict = SlowDict(("localhost", 4543), b"some key")
>       slowdict.start()

wptserve\tests\test_stash.py:121: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
C:\Python27amd64\Lib\multiprocessing\managers.py:524: in start
    self._process.start()
C:\Python27amd64\Lib\multiprocessing\process.py:130: in start
    self._popen = Popen(self)
C:\Python27amd64\Lib\multiprocessing\forking.py:277: in __init__
    dump(process_obj, to_child, HIGHEST_PROTOCOL)
C:\Python27amd64\Lib\multiprocessing\forking.py:199: in dump
    ForkingPickler(file, protocol).dump(obj)
C:\Python27amd64\Lib\pickle.py:224: in dump
    self.save(obj)
C:\Python27amd64\Lib\pickle.py:331: in save
    self.save_reduce(obj=obj, *rv)
C:\Python27amd64\Lib\pickle.py:425: in save_reduce
    save(state)
C:\Python27amd64\Lib\pickle.py:286: in save
    f(self, obj) # Call unbound method with explicit self
C:\Python27amd64\Lib\pickle.py:655: in save_dict
    self._batch_setitems(obj.iteritems())
C:\Python27amd64\Lib\pickle.py:687: in _batch_setitems
    save(v)
C:\Python27amd64\Lib\pickle.py:286: in save
    f(self, obj) # Call unbound method with explicit self
C:\Python27amd64\Lib\multiprocessing\forking.py:67: in dispatcher
    self.save_reduce(obj=obj, *rv)
C:\Python27amd64\Lib\pickle.py:401: in save_reduce
    save(args)
C:\Python27amd64\Lib\pickle.py:286: in save
    f(self, obj) # Call unbound method with explicit self
C:\Python27amd64\Lib\pickle.py:554: in save_tuple
    save(element)
C:\Python27amd64\Lib\pickle.py:286: in save
    f(self, obj) # Call unbound method with explicit self
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <multiprocessing.forking.ForkingPickler instance at 0x000000000578AF08>
obj = <class 'test_stash.SlowDict'>, name = 'SlowDict'
pack = <built-in function pack>

    def save_global(self, obj, name=None, pack=struct.pack):
        write = self.write
        memo = self.memo
    
        if name is None:
            name = obj.__name__
    
        module = getattr(obj, "__module__", None)
        if module is None:
            module = whichmodule(obj, name)
    
        try:
            __import__(module)
            mod = sys.modules[module]
            klass = getattr(mod, name)
        except (ImportError, KeyError, AttributeError):
            raise PicklingError(
                "Can't pickle %r: it's not found as %s.%s" %
>               (obj, module, name))
E           PicklingError: Can't pickle <class 'test_stash.SlowDict'>: it's not found as test_stash.SlowDict

C:\Python27amd64\Lib\pickle.py:754: PicklingError

[gsnedders]

These also seem to fail on macOS on Python 3.8.