Get rid of confusing UnusedArgumentWarning from working code

[jesper-friis]

Description:

Get rid of confusing UnusedArgumentWarning from working code.

Type of change:

• Bug fix and code cleanup.
• New feature.
• Documentation update.
• Testing.

Checklist for the reviewer:

This checklist should be used as a help for the reviewer.

• Is the change limited to one issue?
• Does this PR close the issue?
• Is the code easy to read and understand?
• Do all new feature have an accompanying new test?
• Has the documentation been updated as necessary?

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (6297583) 75.10% compared to head (5ea8c23) 75.17%.

Files	Patch %	Lines
tripper/backends/rdflib.py	75.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #151      +/-   ##
==========================================
+ Coverage   75.10%   75.17%   +0.07%     
==========================================
  Files          17       17              
  Lines        1406     1410       +4     
==========================================
+ Hits         1056     1060       +4     
  Misses        350      350              

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