Bump six from 1.13.0 to 1.15.0

Bumps [six](https://github.com/benjaminp/six) from 1.13.0 to 1.15.0.
- [Release notes](https://github.com/benjaminp/six/releases)
- [Changelog](https://github.com/benjaminp/six/blob/master/CHANGES)
- [Commits](benjaminp/six@1.13.0...1.15.0)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements.txt

@@ -3,7 +3,7 @@ numpy==1.18.1
 scipy==1.4.1
 matplotlib==3.2.1
 scikit-learn==0.22.1
-six==1.13.0
+six==1.15.0
 Pillow==7.1.0
 tqdm==4.45.0
 statsmodels==0.11.0