Cannot pickle reactive classes

[quant1729]

Describe the bug The error is produced if one tries to pickle reactive classes (e.g. Subject). TypeError: cannot pickle '_thread.RLock' object

Maybe it makes sense to add something like:

    def reduce_RLock(obj): return threading.RLock, tuple()
    copyreg.pickle(_thread.RLock, reduce_RLock)

To Reproduce Steps to reproduce the behavior:

1. Create a Subject object
2. Pickle it

Expected behavior The object is pickled