Trivista xml2dict update

[pietsjoh]

Description of the change

The trivista filereader used its own version to convert an ET to a dict.
This PR switches this converter to utils.tools.XmlDict.

Progress of the PR

• Change implemented (can be split into several points),
• add an changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• add tests,
• clarify how to read attrib only (see Unification of XML to dict list tree translation #113 (comment))
• ready for review.

[codecov]

Codecov Report

Attention: Patch coverage is 88.67925% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 85.58%. Comparing base (576e8d9) to head (ab919f1).
Report is 332 commits behind head on main.

Files	Patch %	Lines
rsciio/trivista/_api.py	88.67%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #138      +/-   ##
==========================================
- Coverage   85.64%   85.58%   -0.07%     
==========================================
  Files          74       74              
  Lines        9817     9807      -10     
  Branches     2161     2156       -5     
==========================================
- Hits         8408     8393      -15     
- Misses        909      912       +3     
- Partials      500      502       +2     

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