Hi Leo, thanks for your answer.
The link you indicated are not Trelby sources but the program itself (it should be decompressed and copied to /opt), and does not contain setup.py in the main directory, but only on a sub directory.
The result is the same error, setup.py not found.
I have tried the link to the Trelby sources on github and, in the APKBUILD I uploaded here, even tried to download the sources from a custom github account to check if the problem depended on the link. I always got the same error, .
>>> trelby: Unpacking /var/cache/distfiles/trelby-2.2.tar.gz...
python3: can't open file 'setup.py': [Errno 2] No such file or directory
>>> ERROR: trelby: build failed
Might it be that Trelby requires python2 rather than python3?