html-tidy: 5.4.0 -> 5.6.0

Semi-automatic update. These checks were performed:

- built on NixOS
- ran `/nix/store/9idavy2wphjqap2sw87ivczwq8wy7vjf-html-tidy-5.6.0/bin/tidy -h` got 0 exit code
- ran `/nix/store/9idavy2wphjqap2sw87ivczwq8wy7vjf-html-tidy-5.6.0/bin/tidy --help` got 0 exit code
- ran `/nix/store/9idavy2wphjqap2sw87ivczwq8wy7vjf-html-tidy-5.6.0/bin/tidy help` got 0 exit code
- ran `/nix/store/9idavy2wphjqap2sw87ivczwq8wy7vjf-html-tidy-5.6.0/bin/tidy -V` and found version 5.6.0
- ran `/nix/store/9idavy2wphjqap2sw87ivczwq8wy7vjf-html-tidy-5.6.0/bin/tidy -v` and found version 5.6.0
- ran `/nix/store/9idavy2wphjqap2sw87ivczwq8wy7vjf-html-tidy-5.6.0/bin/tidy --version` and found version 5.6.0
- ran `/nix/store/9idavy2wphjqap2sw87ivczwq8wy7vjf-html-tidy-5.6.0/bin/tidy -h` and found version 5.6.0
- ran `/nix/store/9idavy2wphjqap2sw87ivczwq8wy7vjf-html-tidy-5.6.0/bin/tidy --help` and found version 5.6.0
- found 5.6.0 with grep in /nix/store/9idavy2wphjqap2sw87ivczwq8wy7vjf-html-tidy-5.6.0
- found 5.6.0 in filename of file in /nix/store/9idavy2wphjqap2sw87ivczwq8wy7vjf-html-tidy-5.6.0

cc "@edwtjo"