Corrected literals in SELECT and CONSTRUCT queries in the sparqlwrapper backend

[jesper-friis]

Description

Note: this fix builds on top of PR #331

Closes #320

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?
• Is the code properly tested?

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 76.67%. Comparing base (6541542) to head (bdf0e3e).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
tripper/backends/sparqlwrapper.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #335   +/-   ##
=======================================
  Coverage   76.66%   76.67%           
=======================================
  Files          22       22           
  Lines        2507     2508    +1     
=======================================
+ Hits         1922     1923    +1     
  Misses        585      585           

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