Added support for xsd:nonNegativeInteger literals

[jesper-friis]

Description:

• Added support for xsd:nonNegativeInteger, since they occurs in cardinality restrictions.
• Moved consistency checking of literals to the Literal constructor instead of in the to_python() method.
• Added more tests to improve test coverage
• Added tests for warnings (and ignored those coming from other packages)

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]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 77.40%. Comparing base (9271898) to head (866e3ed).

Files	Patch %	Lines
tripper/literal.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #178      +/-   ##
==========================================
+ Coverage   76.82%   77.40%   +0.57%     
==========================================
  Files          17       17              
  Lines        1536     1540       +4     
==========================================
+ Hits         1180     1192      +12     
+ Misses        356      348       -8     

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