`"is" with literal` warning

[JulianKnodt]

https://github.com/HTDerekLiu/BlenderToolbox/blob/11901ca6a9c772107c7f38b490d0fc3788b99337/blendertoolbox/setMeshColors.py#L55

There's a few spots where equality between strings is checked using is, which I think may lead to incorrect output (and is also just annoying to get the warnings).

I can submit a PR later to update it.