sylpheed: 3.6.0 -> 3.7.0

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

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

These checks were done:

- built on NixOS
- ran `/nix/store/h2523jz7ladr0j6m3v6a0x45ak0z9iq8-sylpheed-3.7.0/bin/sylpheed --version` and found version 3.7.0
- found 3.7.0 with grep in /nix/store/h2523jz7ladr0j6m3v6a0x45ak0z9iq8-sylpheed-3.7.0
- directory tree listing: https://gist.github.com/8f404aeeccf14771245b2a25b3a1231e