Replace PyYAML import with ruamel.yaml in test_common_io.py

[rly]

Summary

tests/unit/common/test_common_io.py uses import yaml (PyYAML), but pyyaml is not a declared dependency of hdmf. The import works in pip-based environments because pip's h5py wheel transitively depends on pyyaml, but conda's h5py package does not have this transitive dependency, causing:

ModuleNotFoundError: No module named 'yaml'

This is blocking the conda-forge feedstock release: conda-forge/hdmf-feedstock#102

Failure log: https://dev.azure.com/conda-forge/feedstock-builds/_build/results?buildId=1479230&view=logs&jobId=7b6f2c87-f3a7-5133-8d84-7c03a75d9dfc&j=7b6f2c87-f3a7-5133-8d84-7c03a75d9dfc&t=9eb77fd2-8ddd-5444-8fc0-71cb28dcb736

Fix

Replace import yaml / yaml.safe_load() with ruamel.yaml, which is already a declared core dependency and used throughout the codebase for YAML loading.