source=(https://bitbucket.org/nikratio/s3ql/downloads/s3ql-$pkgver.tar.bz2) The source contains a src/s3ql/deltadump.c generated from a src/s3ql/deltadump.pyx
The .pyx is part of the S3QL codebase. The .c is auto-generated using Cython 0.25.2; it's probably included over being built with makepkg to avoid the Cython dependency.
Python 3.7 made changes which broke Cython prior to 0.28 (or <=0.27.2 or something). The generated 0.25.2 code definitely doesn't work anymore with Python3.7. As such, that source file with the generated Cython 0.25.2 file are not compatible with Python 3.7 and this package is "broken".
The fix would be to run: cython src/s3ql/deltadump.pyx (or not include generated code).