style: fix/ignore flake8 errors in store related code

[aucampia]

Summary of changes

This patch adds ignores for flake8 errors in store related code, it also
removes one unused import in rdflib/plugins/stores/sparqlconnector.py.

Mainly doing this in preparation for adding typing to rdflib/store.py which will invalidate large parts of the flakeheaven baseline there.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

Coverage remained the same at 88.514% when pulling ae5771d on aucampia:iwana-20220522T2220-fix_flake8_stores into 958b9a1 on RDFLib:master.

[ghost]

lgtm