Allow prefixes argument of expand_iri() to map to a Namespace object.

[jesper-friis]

Description

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

❌ Patch coverage is 92.30769% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.55%. Comparing base (5e2fcda) to head (58a3465).

Files with missing lines	Patch %	Lines
tripper/utils.py	84.61%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #563   +/-   ##
=======================================
  Coverage   85.54%   85.55%           
=======================================
  Files          30       30           
  Lines        5121     5142   +21     
=======================================
+ Hits         4381     4399   +18     
- Misses        740      743    +3     

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[francescalb]

Approved by Jesper, but since he created the PR he cannot do it formally. Joint work by him and me.