Fix issue with cartridge_icon parsing from xml.

[cutz]

Fix tag name for cartridge_icon. Fix attribute error.

[codecov[bot]]

Codecov Report

Merging #58 into master will increase coverage by 0.61%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
+ Coverage   84.86%   85.47%   +0.61%     
==========================================
  Files          16       16              
  Lines         654      654              
  Branches      114      114              
==========================================
+ Hits          555      559       +4     
+ Misses         77       75       -2     
+ Partials       22       20       -2

Impacted Files	Coverage Δ
src/lti/tool_config.py	90.29% <100%> (+2.98%)	:arrow_up:

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[cutz]

Thanks!