Switch all uses of pybind's py::enum_ to py::native_enum.

[virajbshah]

• Switches the following uses of the now deprecated py::enum_ to py::native_enum:
  • basic_block.OperandType
  • graph_builder.NodeType
  • graph_builder.EdgeType
  • oov_token_behavior.BehaviorType
  • bhive_to_exegesis.AnnotatorType
• Updates any usages of these types to take advantage of the full native Enum interface.
• The downside is that py::native_enum does not take an Enum-level docstring. Member-level docstrings are kept.

[virajbshah]

It seems like bumping pybind11 causes some incompatibility with TensorFlow's usage of pybind11_absl. It'll probably take some investigating to figure out.