Ensure that SPARQL queries with the rdflib backend behaves similar to sparqlwrapper

[jesper-friis]

Description

Align the rdflib backend with sparqlwrapper.

The following was done:

• Ensure that literals returned by Triplestore.query() with the rdflib backend are converted to tripper.Literal and not strings. This will make rdflib behaving similar to sparqlwrapper.
• Deprecated Triplestore.available() and replaced it with the new Triplestore.is_available() that works for all backends.
• For consistency with base_iri and update_iri, renamed check_url to check_iri. Deprecated the use of check_url .

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.04%. Comparing base (c71f765) to head (8848476).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
tripper/backends/sparqlwrapper.py	85.71%	1 Missing ⚠️
tripper/triplestore.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #456      +/-   ##
==========================================
- Coverage   84.06%   84.04%   -0.03%     
==========================================
  Files          30       30              
  Lines        4167     4186      +19     
==========================================
+ Hits         3503     3518      +15     
- Misses        664      668       +4     

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