Fix PEP 604 union types breaking when using Class Signatures for ChainOfThought (#8465)

Author: erandeutsch

dspy/signatures/signature.py

@@ -430,7 +430,7 @@ class MyType:
         # program of thought and teleprompters, so we just silently default to string.
         if type_ is None:
             type_ = str
-        if not isinstance(type_, (type, typing._GenericAlias, types.GenericAlias, typing._SpecialForm)):
+        if not isinstance(type_, (type, typing._GenericAlias, types.GenericAlias, typing._SpecialForm, types.UnionType)):
             raise ValueError(f"Field types must be types, but received: {type_} of type {type(type_)}.")
         if not isinstance(field, FieldInfo):
             raise ValueError(f"Field values must be Field instances, but received: {field}.")