Bump six from 1.14.0 to 1.15.0

Bumps [six](https://github.com/benjaminp/six) from 1.14.0 to 1.15.0.
- [Release notes](https://github.com/benjaminp/six/releases)
- [Changelog](https://github.com/benjaminp/six/blob/master/CHANGES)
- [Commits](https://github.com/benjaminp/six/compare/1.14.0...1.15.0)

requirements.txt

@@ -28,7 +28,7 @@ pytz==2019.3
 PyWavelets==1.1.1
 scikit-learn==0.22.1
 scipy==1.4.1
-six==1.14.0
+six==1.15.0
 wcwidth==0.1.8
 zipp==2.2.0
 astor==0.8.1