Fix access to proto enum fields

[dsmilkov]

Fix access to the proto enum fields. Only newest protobuf version allows enum constants to be accessed via the enum name. However, if users have already installed an older version of protobuf which satisfies requirement constraints of tensorflowjs, they will get the following error:

File "/usr/local/lib/python3.6/dist-packages/tensorflowjs/converters/fuse_prelu.py", line 42, in register_prelu_op
value.list.type.extend([types_pb2.DataType.DT_FLOAT])
AttributeError: 'EnumTypeWrapper' object has no attribute 'DT_FLOAT'

Also freeze the version of a transitive dependency, gast. gast 0.3 introduced breaking API changes and the python TensorFlow requirement is relaxed.

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This change is 

[nsthorat]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained