Fallback to kernelspec to check if it's a Python notebook (#12875)
## Summary
This PR adds a fallback logic for `is_python_notebook` to check the
`kernelspec.language` field.
Reference implementation in VS Code:
https://github.com/microsoft/vscode/blob/1c31e758985efe11bc0453a45ea0bb6887e670a4/extensions/ipynb/src/deserializers.ts#L20-L22
It's also required for the kernel to provide the `language` they're
implementing based on
https://jupyter-client.readthedocs.io/en/stable/kernels.html#kernel-specs
reference although that's for the `kernel.json` file but is also
included in the notebook metadata.
Closes: #12281
## Test Plan
Add a test case for `is_python_notebook` and include the test notebook
for round trip validation.
The test notebook contains two cells, one is JavaScript (denoted via the
`vscode.languageId` metadata) and the other is Python (no metadata). The
notebook metadata only contains `kernelspec` and the `language_info` is
absent.
I also verified that this is a valid notebook by opening it in Jupyter
Lab, VS Code and using `nbformat` validator.
