fix(core): support unpickling older pickles

The changes to pickling in 6a1c295 means that
in sympy>=1.9 Symbols are pickled using __getnewargs_ex__ rather than
__getstate__/__setstate__. This breaks unpickling of pickles created with
sympy<1.9 because those pickle files still need to use __setstate__ when being
unpickled.

This commit adds back the __setstate__ method to Symbol although it should only
be used when unpickling pickles created in older SymPy versions.

sympy/core/symbol.py

@@ -304,6 +304,15 @@ def __new_stage2__(cls, name, **assumptions):
     def __getnewargs_ex__(self):
         return ((self.name,), self.assumptions0)
 
+    # NOTE: __setstate__ is not needed for pickles created by __getnewargs_ex__
+    # but was used before Symbol was changed to use __getnewargs_ex__ in v1.9.
+    # Pickles created in previous SymPy versions will still need __setstate__
+    # so that they can be unpickled in SymPy > v1.9.
+
+    def __setstate__(self, state):
+        for name, value in state.items():
+            setattr(self, name, value)
+
     def _hashable_content(self):
         # Note: user-specified assumptions not hashed, just derived ones
         return (self.name,) + tuple(sorted(self.assumptions0.items()))