Downgrade log message about plugin without override argument to debug from warning

[yarikoptic]

Summary of changes

Original issue

• Addition of override argument to rdflib.store.Store.bind breaks compatibility #1880

was worked around by providing a fall back call without "override" in

• Fallback to old Store.bind signature on TypeError #2018

As a result any user with such a store (in our case just <class 'rdflib.plugins.memory.IOMemory'>) is flooded with such warnings pointing to that issue #1880 and for all means and purposes AFAIK just need to ignore them since there is nothing user can do to address it.

That raises the question on why is it a Warning at a user level and not some debug message for developers/troubleshooting etc? Hence I am lowering it to debug level to bring peace of mind to users of the library.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• If the change has a potential impact on users of this project:
  • Added or updated tests that fail without the change.
  • Updated relevant documentation to avoid inaccuracies.
  • Considered adding additional documentation.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

coverage: 90.334%. remained the same
when pulling e336421 on yarikoptic:rf-no-warning
into 62c528d on RDFLib:main.