[Feature]: Clean up HDMFIO.load_namespaces call

[rly]

What would you like to see added to PyNWB?

Resolve TODO in #1911 (comment) where the class method HDMFIO.load_namespaces requires passing io._file.

Is your feature request related to a problem?

hdmf-dev/hdmf-zarr#293

What solution would you like?

I think an HDMFIO instance should be able to call load_namespaces on the open file handled by itself, regardless of backend, so the io._file call (which does not work consistently between HDF5IO and ZarrIO) is not needed.

Do you have any interest in helping implement the feature?

Yes.

Code of Conduct

• I agree to follow this project's Code of Conduct
• Have you checked the Contributing document?
• Have you ensured this change was not already requested?

[rly]

This depends on changes in hdmf and hdmf-zarr linked above to be merged first