python27Packages.pyunbound: 1.6.0 -> 1.7.3 (#42952)

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

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

These checks were done:

- built on NixOS

- 0 of 0 passed binary check by having a zero exit code.
- 0 of 0 passed binary check by having the new version present in output.
- found 1.7.3 with grep in /nix/store/1m9fiqaz72ry8jkwgw52v3wv1n5pgx0l-pyunbound-1.7.3
- directory tree listing: https://gist.github.com/2a7b7a3d8411e63dec56620537e2d00f
- du listing: https://gist.github.com/82633a34688c0630480a740b7562ac4c