Bump pillow from 5.1.0 to 6.2.0 in /cvat/requirements (#808)

Bumps [pillow](https://github.com/python-pillow/Pillow) from 5.1.0 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](https://github.com/python-pillow/Pillow/compare/5.1.0...6.2.0)

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

cvat/requirements/base.txt

@@ -7,7 +7,7 @@ django-compressor==2.2
 django-rq==2.0.0
 EasyProcess==0.2.3
 ffmpy==0.2.2
-Pillow==5.1.0
+Pillow==6.2.0
 numpy==1.16.2
 patool==1.12
 python-ldap==3.0.0