Emboss fails to run on Python 3.9

In addition to #136 , Emboss fails to run on Python 3.9, which unlike 3.8 is not near the end of the support horizon. The failures appear to be related to typechecking:

```python
/home/jason/.cache/bazel/_bazel_jason/3f005b1d454ca9fd185dd4ffbad81e8e/sandbox/linux-sandbox/1366/execroot/com_google_emboss/bazel-out/k8-fastbuild/bin/compiler/back_end/cpp/header_generator_test.runfiles/com_google_emboss/compiler/util/ir_data_fields.py", line 290, in
def field_specs(obj: IrDataT | type[IrDataT]) -> Mapping[str, FieldSpec]:
TypeError: unsupported operand type(s) for |: 'TypeVar' and 'types.GenericAlias'
```