Bump pillow from 5.3.0 to 6.2.0

Bumps [pillow](https://github.com/python-pillow/Pillow) from 5.3.0 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](python-pillow/Pillow@5.3.0...6.2.0)

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

requirements.txt

@@ -16,7 +16,7 @@ idna==2.7
 jmespath==0.9.3
 numpy==1.15.4
 oauthlib==2.1.0
-Pillow==5.3.0
+Pillow==6.2.0
 pyparsing==2.3.0
 PySocks==1.6.8
 pytest==4.0.1