Fix missing Python coverage in Codecov reports

[JeanChristopheMorinPerso]

Fixes #1039

Use one line for each Issue. This allows auto-closing the related issue when the fix is merged.

Summary

As stated in #1039 , with the recent update of the codecov/codecov-action GH action, the Python coverage is missing from the Codecov reports.

This is due to the fact that the old Codecov action was using a different uploader script that was taking care of calling coverage xml when it found Python coverage files. The new action doesn't seem to do it.

So the fix is as simple as calling coverage xml from the make coverage target.

I've also added an rm in the lcov target because the C++ coverage files coverage.filtered.info and coverage.info were both uploaded which waas basically undoing the filtering we were doing in the lcov target. Though, I see that there is also some filtering in .codecov.yml, but it doesn't seems to be applied... Not too sure why.

[codecov-commenter]

Codecov Report

Merging #1040 (f93aab8) into master (aada787) will increase coverage by 3.52%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1040      +/-   ##
==========================================
+ Coverage   82.78%   86.31%   +3.52%     
==========================================
  Files          75      190     +115     
  Lines        3306    18963   +15657     
  Branches        0     2099    +2099     
==========================================
+ Hits         2737    16367   +13630     
- Misses        569     2050    +1481     
- Partials        0      546     +546     

Flag	Coverage Δ
unittests	86.31% <ø> (+3.52%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
tests/test_cmx_3600_adapter.py	99.03% <0.00%> (ø)
...-opentimelineio/opentimelineio/plugins/manifest.py	86.56% <0.00%> (ø)
tests/baselines/example_schemadef.py	85.71% <0.00%> (ø)
tests/test_filter_algorithms.py	98.89% <0.00%> (ø)
.../opentimelineio/schema/image_sequence_reference.py	100.00% <0.00%> (ø)
...y-opentimelineio/opentimelineio/schema/timeline.py	100.00% <0.00%> (ø)
tests/test_marker.py	94.28% <0.00%> (ø)
...timelineio/console/autogen_plugin_documentation.py	72.72% <0.00%> (ø)
.../opentimelineio_contrib/adapters/ffmpeg_burnins.py	0.00% <0.00%> (ø)
tests/test_cdl.py	95.00% <0.00%> (ø)
... and 105 more

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[JeanChristopheMorinPerso]

For transparency: I've let this PR as a draft while I find time to investigate a little bit more. I'd like to get to the bottom of why the ignore list in .codecov.yaml doesn't seems to have any effect.

If anyone wants to participate/help, feel free to reply :)

I was also unable to produce C++ coverage on my Linux laptop, which annoys me quite a lot.

[JeanChristopheMorinPerso]

This is now ready to be reviewed.

[ssteinbach]

Looks great, thanks!