pynac: 0.7.19 -> 0.7.21 (#41662)

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

This update was made based on information from https://repology.org/metapackage/pynac/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 0.7.21 with grep in /nix/store/883yhmfc1k4vz1dd37l73v7clvnyaa3v-pynac-0.7.21
- directory tree listing: https://gist.github.com/a0f53cebb1c4cae07c81edef938ac632
- du listing: https://gist.github.com/78800724710d0766e4add7ed1847c3fc