[pypy-dev] Release v7.3.15 of PyPy

I have prepared a new release of PyPy. This is mainly a bugfix release to fix backward compatibility in c-extensions that use the buffer protocol. It also includes chages from our move to Git/Github. The pre-release tarballs are now on https://buildbot.pypy.org/pypy/ (until the CDN percolates them out to https://downloads.python.org/pypy/), the checksums are on a Pr to pypy.org at https://deploy-preview-110--keen-mestorf-442210.netlify.app/checksums. The release notes are at https://doc.pypy.org/en/latest/release-v7.3.15.html. Please test out the source tarballs, they have changed since they are from a git repo. I plan to release officially later today. Matti