xonsh: 0.6.6 -> 0.6.7

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/xonsh/versions.

These checks were done:

- built on NixOS
- Warning: no invocation of /nix/store/mb1vva59dwhq6yzqf98gkygljbzpdf37-xonsh-0.6.7/bin/xon.sh had a zero exit code or showed the expected version
- /nix/store/mb1vva59dwhq6yzqf98gkygljbzpdf37-xonsh-0.6.7/bin/.xonsh-wrapped passed the binary check.
- /nix/store/mb1vva59dwhq6yzqf98gkygljbzpdf37-xonsh-0.6.7/bin/xonsh passed the binary check.
- 2 of 3 passed binary check by having a zero exit code.
- 2 of 3 passed binary check by having the new version present in output.
- found 0.6.7 with grep in /nix/store/mb1vva59dwhq6yzqf98gkygljbzpdf37-xonsh-0.6.7
- directory tree listing: https://gist.github.com/1d8694bad62c300714d6658deedc7ec0
- du listing: https://gist.github.com/2d8baff97fce82b5ba4ded4746d954a0