Add Symbol serializability

Symbol was not (un)picklable as Symbol.__getattr__ was getting in the way while looking for __setstate__.

symbolite/abstract/symbol.py

@@ -120,9 +120,11 @@ def __getitem__(self, key: Any) -> Self:
         return getitem(self, key)
 
     # Emulating attribute
-    def __getattr__(self, key: Any) -> Self:
+    def __getattr__(self, key: str) -> Self:
         """Defines behavior for when an item is accessed,
         using the notation self.key"""
+        if key.startswith("__"):
+            raise AttributeError(key)
         return symgetattr(self, key)
 
     # Normal arithmetic operators

symbolite/testsuite/test_serialize.py

@@ -0,0 +1,13 @@
+import pickle
+
+from pytest import mark
+
+from .. import Scalar, Symbol, Vector
+
+
+@mark.parametrize("cls", [Scalar, Symbol, Vector])
+def test_pickle(cls):
+    obj = cls()
+    dump = pickle.dumps(obj)
+    load = pickle.loads(dump)
+    assert load == obj