fix build

Merge pull request mikedh#1453 from mikedh/fix/flake8concat

Flake8 Rule

Merge pull request mikedh#1449 from friggog/negative-obj-indices

Support negative OBJ indices

Merge pull request mikedh#1447 from mikedh/fix/daemat

Fix: DAE import-export materials

exclude mac/2.7 from CI

Merge pull request mikedh#1427 from mikedh/feat/proj

Release: Scene Fixes

Merge pull request mikedh#1398 from mikedh/release/subscene

Release: Subscene

Merge pull request mikedh#1382 from mikedh/fix/dxf

Release: DXF export

Merge pull request mikedh#1374 from mikedh/fix/pbrhash

Release: PBR Hash Fixes And Submesh Slicing

Merge pull request mikedh#1357 from mikedh/merge/fixes

Release: Visual Fixes