Add better and more convenient support for rdf:JSON datatype

[jesper-friis]

Description

This PR makes it easy to represent JSON-serialisable Python objects (like lists and dicts) as literals of datatype rdf:JSON. The following is now possible:

>>> from tripper import Literal
>>> literal = Literal({'a': 1, 'b': 2})
>>> literal.value
{'a': 1, 'b': 2}
>>> literal.n3()
'"{"a": 1, "b": 2}"^^<http://www.w3.org/1999/02/22-rdf-syntax-ns#JSON>'

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

All modified and coverable lines are covered by tests ✅

Project coverage is 78.35%. Comparing base (2172204) to head (a398386).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #228      +/-   ##
==========================================
+ Coverage   78.16%   78.35%   +0.19%     
==========================================
  Files          18       18              
  Lines        1708     1723      +15     
==========================================
+ Hits         1335     1350      +15     
  Misses        373      373              

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[francescalb]

good.