Replace deprecated method in csv2rdf

[ageorgou]

Summary of changes

Fixes #2618.

The code for tne csv2rdf command-line tool calls ConfigParser.readfp, which has been deprecated for a while and has been removed in Python 3.12. The read_file method is a straightforward replacement.

I have checked manually that the old and new version give the same result for a sample config file.

I've also taken the opportunity to add another small test, since the coverage for that tool is quite low. The new test doesn't check that the contents of the config file are processed correctly, only that it is opened.

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:
  • No change besides future-proofing.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

coverage: 90.651% (+0.02%) from 90.628%
when pulling 290d766 on ageorgou:csv2rdf-fix
into 0a0d4b6 on RDFLib:main.

[ashleysommer]

@ageorgou
Thank you for this fix.