Pin typing extensions. (apple#343)

pyproject.toml

@@ -33,7 +33,7 @@ dependencies = [
     "tensorflow_text==2.14.0; platform_machine == 'x86_64'", # implied by seqio, but also used directly for text processing
     "tensorstore>=0.1.21",  # used for supporting GDA checkpoints
     "toml",  # for config management
-    "typing-extensions>=4.2.0",  # needed for typing.Protocol
+    "typing-extensions==4.9.0",  # needed for typing.Protocol. >4.9.0 runs into attribute error `__non_callable_proto_members__`.
     "seqio==0.0.18",  # used for inputs
 ]