More explicit testing of pickling of user defined symbol

[bjodah]

This draft-PR tracks symengine/symengine#2129

[bjodah]

So when using:
symengine/symengine@d10ee0b

we get this pytest failure consistently:

________________________________ test_pysymbol _________________________________

    def test_pysymbol():
        a = MySymbol("hello", attr=1)
        b = pickle.loads(pickle.dumps(a + 2)) - 2
        c = MySymbol("hello", attr=-1)
        try:
>           assert a == b
E           assert hello == hello

symengine/tests/test_pickling.py:43: AssertionError

(b is no longer a MySymbol instance but simply an instance of Symbol). As to why this happens: I'm currently clueless, my understanding was that the information regarding the python instance would be by the pickling mechanism (i.e. outside of the serialized c++ representation), but clearly I'm missing something.

[bjodah]

Nevermind, here's the reason:

symengine.py/symengine/lib/pywrapper.cpp

Line 327 in 4f0775a

void save_basic(RCPBasicAwareOutputArchive<cereal::PortableBinaryOutputArchive> &ar, const Symbol &b)