[FLINK-21724][python] Remove type annotation from python message.as_type

This closes #213
diff --git a/statefun-python-sdk/statefun/messages.py b/statefun-python-sdk/statefun/messages.py
index cda78ff..bd76ce2 100644
--- a/statefun-python-sdk/statefun/messages.py
+++ b/statefun-python-sdk/statefun/messages.py
@@ -92,7 +92,7 @@
         tv = self.typed_value
         return tv.value if tv.has_value else None
 
-    def as_type(self, tpe: Type) -> typing.Optional[bytes]:
+    def as_type(self, tpe: Type):
         tv = self.typed_value
         if tv.has_value:
             serializer = tpe.serializer()