Add DXF import to master

[adam-urbanczyk]

Follow-up of #351

[adam-urbanczyk]

@jmwright could you merge this once CI becomes green?

[codecov]

Codecov Report

Merging #372 into master will decrease coverage by 0.05%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master     #372      +/-   ##
==========================================
- Coverage   95.57%   95.52%   -0.06%     
==========================================
  Files          19       19              
  Lines        4700     4777      +77     
==========================================
+ Hits         4492     4563      +71     
- Misses        208      214       +6     

Impacted Files	Coverage Δ
cadquery/occ_impl/importers.py	91.56% <89.83%> (-4.74%)	⬇️
cadquery/cq.py	94.09% <100.00%> (+0.02%)	⬆️
tests/test_importers.py	96.00% <100.00%> (+2.06%)	⬆️

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[jmwright]

@adam-urbanczyk Done