force dxf layer name case agreement

[greyltc]

fixes #624

[jmwright]

@greyltc Please have a look at the black lint check error when you get a chance.

[codecov]

Codecov Report

Merging #630 (5ffb1af) into master (b6beba7) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #630      +/-   ##
==========================================
+ Coverage   93.93%   93.94%   +0.01%     
==========================================
  Files          31       31              
  Lines        6526     6543      +17     
  Branches      712      713       +1     
==========================================
+ Hits         6130     6147      +17     
  Misses        258      258              
  Partials      138      138              

Impacted Files	Coverage Δ
cadquery/occ_impl/importers.py	88.42% <100.00%> (+0.09%)	⬆️
tests/test_cadquery.py	99.09% <0.00%> (+<0.01%)	⬆️
cadquery/cq.py	89.66% <0.00%> (+0.08%)	⬆️

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[jmwright]

Merging. Thanks @greyltc !