xmlrunner: Expose python test class docstrings as comments in the XML result file

[oliviermartin]

In the same principle as the PR I sent 2 years ago: https://github.com/xmlrunner/unittest-xml-reporting/pull/214 But this time is to add comments to 'testsuite' XML nodes.

• xmlrunner: Expose python test class docstrings as comments in the XML result file

These comments could for instance be retrieved by XSLT to add description to test suite.

• tests: Added Python Docstring to test new support

[dnozay]

@oliviermartin , PR looks okay but it would be good to have a testcase to leverage these.

[codecov-commenter]

Codecov Report

Merging #270 (18792dd) into master (3a2be20) will decrease coverage by 0.17%. The diff coverage is 72.72%.

@@            Coverage Diff             @@
##           master     #270      +/-   ##
==========================================
- Coverage   94.87%   94.70%   -0.18%     
==========================================
  Files          12       12              
  Lines        1542     1549       +7     
==========================================
+ Hits         1463     1467       +4     
- Misses         79       82       +3     

Impacted Files	Coverage Δ
xmlrunner/result.py	97.82% <72.72%> (-0.79%)	:arrow_down:

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