Tests for GimpGradientFile and a sample .ggr by me
Add (failing) test for GIMP 1.3 gradient files and .ggr by me
Fix for GIMP 1.3 gradient files
flake8
Test GimpGradientFile.sine()
Fix for Python 3
Merge pull request #874 from hugovk/GimpGradientFile

Tests for GimpGradientFile