I'm having a hard time understanding the workflow you intended for this PKGBUILD. As I picture the use of this script, you'd have to get a source tarball somehow, stick it in the same directory as this PKGBUILD, then run this. The PKGBUILD usually gets the source tarball for you though. Plus, I reckon that an arch package should be put in AUR rather than the upstream package (and it already is in AUR with the cleanly-written pybitmessage-git package)... to me one of the main points of arch is to avoid polluting upstream codebases with special packaging.