Fix mutable serializer type hint

Summary: Fix the mutable type serializer typing, `GeneratedError` -> `MutableGeneratedError`

Reviewed By: createdbysk

Differential Revision: D66938381

fbshipit-source-id: 452931b02421e8157022954714b9593107bcd06b

third-party/thrift/src/thrift/lib/python/mutable_serializer.pyi

@@ -17,11 +17,13 @@
 import typing
 
 from folly.iobuf import IOBuf
-from thrift.python.exceptions import GeneratedError
+from thrift.python.mutable_exceptions import MutableGeneratedError
 from thrift.python.mutable_types import MutableStructOrUnion
 from thrift.python.protocol import Protocol as _Protocol
 
-sT = typing.TypeVar("sT", bound=typing.Union[MutableStructOrUnion, GeneratedError])
+sT = typing.TypeVar(
+    "sT", bound=typing.Union[MutableStructOrUnion, MutableGeneratedError]
+)
 
 Protocol = _Protocol