dev-libs/libsigc++-2 is no longer required by skype since version 2.0.0.13. Source: https://developer.skype.com/LinuxSkype/ReleaseNotes ldd on the skype binary confirms this.
Version 2.0.0.63 still does not need dev-libs/libsigc++-2, so dependency can be removed.
Fixed in tree. Thanks for the report.