Fix dxf arc export for some partial circles

[snagy]

Hey folks!

I'm not sure if this is the 'right' fix, but some partial circles (specifically, the ones with c_dz.XYZ().Z() < 0) were exporting flipped for me.

The changes are pretty minor, and fixed all the broken arcs on my data. Basically, rotate the arc 180 degrees. I also flipped the start/end and used the is_counterclockwise flag for those arcs.

Broken:

Fixed:

[adam-urbanczyk]

Thanks a lot @snagy ! Could you fix the formatting (black is complaining in CI)? It would be also great if you could share some code that triggers this issue - maybe we could add it to the tests.

[snagy]

@adam-urbanczyk done and done!

[codecov]

Codecov Report

Merging #455 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #455   +/-   ##
=======================================
  Coverage   93.61%   93.62%           
=======================================
  Files          25       25           
  Lines        5326     5332    +6     
  Branches      554      554           
=======================================
+ Hits         4986     4992    +6     
  Misses        215      215           
  Partials      125      125           

Impacted Files	Coverage Δ
cadquery/occ_impl/exporters/dxf.py	92.85% <100.00%> (ø)
tests/test_exporters.py	100.00% <100.00%> (ø)

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[adam-urbanczyk]

Looks good, thanks @snagy !