pythonPackages.thumbor: 6.4.2 -> 6.5.1

The current `thumbor` build on master: https://hydra.nixos.org/build/76000076

This is mainly caused by several dependency bumps (e.g.
`pythonPackages.tornado` to v5 in dd936ccd)
that broke the `setup.py` from version 6.4.

Bumping to the latest stable version (6.5.1) fixes these issues.