mkdocs: 0.17.2 -> 0.17.3

Semi-automatic update generated by https://github.com/ryantm/nix-update tools. These checks were done:

- built on NixOS
- ran `/nix/store/wy5434b824p55fr8x67vz8q8ph9g65kn-mkdocs-0.17.3/bin/.mkdocs-wrapped -h` got 0 exit code
- ran `/nix/store/wy5434b824p55fr8x67vz8q8ph9g65kn-mkdocs-0.17.3/bin/.mkdocs-wrapped --help` got 0 exit code
- ran `/nix/store/wy5434b824p55fr8x67vz8q8ph9g65kn-mkdocs-0.17.3/bin/.mkdocs-wrapped -V` and found version 0.17.3
- ran `/nix/store/wy5434b824p55fr8x67vz8q8ph9g65kn-mkdocs-0.17.3/bin/.mkdocs-wrapped --version` and found version 0.17.3
- ran `/nix/store/wy5434b824p55fr8x67vz8q8ph9g65kn-mkdocs-0.17.3/bin/mkdocs -h` got 0 exit code
- ran `/nix/store/wy5434b824p55fr8x67vz8q8ph9g65kn-mkdocs-0.17.3/bin/mkdocs --help` got 0 exit code
- ran `/nix/store/wy5434b824p55fr8x67vz8q8ph9g65kn-mkdocs-0.17.3/bin/mkdocs -V` and found version 0.17.3
- ran `/nix/store/wy5434b824p55fr8x67vz8q8ph9g65kn-mkdocs-0.17.3/bin/mkdocs --version` and found version 0.17.3
- found 0.17.3 with grep in /nix/store/wy5434b824p55fr8x67vz8q8ph9g65kn-mkdocs-0.17.3
- directory tree listing: https://gist.github.com/b9a97908c199575f84016b19365ae126