Motivation for this change

Currently, the coz package installs the coz script directly, which uses the environment's Python. This should make the closure depend on Python 3 and fixes the shebang.

Note: while the script uses Python2.7, the README says that Python 3 is recommended, and I've tested the script with Python 3 a bit, and seems to run just fine.

This also removes the unset of PYTHONPATH, which is not done by wrapPythonPrograms, so I'm paging @zimbatm to ask if he still has the same issue with this method.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.