Bump pillow from 4.2.1 to 6.2.0

Bumps [pillow](https://github.com/python-pillow/Pillow) from 4.2.1 to 6.2.0.
- [Release notes](https://github.com/python-pillow/Pillow/releases)
- [Changelog](https://github.com/python-pillow/Pillow/blob/master/CHANGES.rst)
- [Commits](https://github.com/python-pillow/Pillow/compare/4.2.1...6.2.0

)
Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -8,7 +8,7 @@ numpy==1.14.3
 olefile==0.45.1
 packaging==16.8
 pandas==0.24.2
-Pillow==4.2.1
+Pillow==6.2.0
 pycosat==0.6.3
 pycparser==2.18
 pycrypto==2.6.1