Remove SupySandbox

This can always be recovered with git. I am removing this, because this
causes errors in the Travis CI build and pysandbox's readme has the
following notice:

```
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!! WARNING WARNING WARNING WARNING WARNING WARNING WARNING WARNING !!!
!!!                                                                 !!!
!!! pysandbox is BROKEN BY DESIGN, please move to a new sandboxing  !!!
!!! solution: run python in a sandbox, not the opposite!            !!!
!!!                                                                 !!!
!!! Learn more about pysandbox failure:                             !!!
!!! https://lwn.net/Articles/574215/                                !!!
!!!                                                                 !!!
!!! WARNING WARNING WARNING WARNING WARNING WARNING WARNING WARNING !!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
```