Do not consider method-lines as relevant in cobertura

[Swatinem]

The etree API is a bit weird, because the iter() method will iterate over matching elements in the whole subtree, instead of only over child elements. Which we actually use extensively, as we are not fully traversing the xml structure of the various formats, but just iterate over the relevant elements, regardless of their nesting.

However for cobertura this might not be the right thing to do. As a class can have lines>line, but also a class>methods>method can have lines>line as well. We are only interested in the directly declared lines>line, not the ones nested within methods however.

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Along with fixing this for the cobertura processor, this also changes some tests to use the proper class. We use the lxml library in production, but a ton of tests were using the xml library, which has a mostly compatible API, but differs in some small details. For example, lxml has the iterchildren method, while xml does not.

Along with changing to the proper lxml library, some tests also changed their inputs to bytes, as the lxml library is otherwise throwing an error when trying to parse an xml declaration that defines an encoding from a python str, which is utf-8 by definition.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.72%. Comparing base (725a8d1) to head (cbc8484).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1170      +/-   ##
==========================================
- Coverage   97.72%   97.72%   -0.01%     
==========================================
  Files         449      449              
  Lines       36864    36866       +2     
==========================================
+ Hits        36026    36027       +1     
- Misses        838      839       +1     

Flag	Coverage Δ
integration	42.89% <71.42%> (+<0.01%)	⬆️
unit	90.42% <100.00%> (-0.01%)	⬇️

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[codecov-notifications]

Codecov Report

All modified and coverable lines are covered by tests ✅

✅ All tests successful. No failed tests found.

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