The user contacted me and said that py-sip dependency was missing.
Changes:
* Added the `py-sip` dependency (thanks to the user, Goran Mekić <meka@tilda.center>, for notifying me)
* Added `USES=shebangfix` because it is needed now
* Added `CONFLICTS_BUILD=rtaudio-* rtmidi-*` because it is bundled with incompatible versions of both packages
and the build picks the preinstalled headers/libs first