* Update PyPy download links. Remove releases that are no longer available.
PyPy has moved from bitbucket.org to foss.heptapod.net.
Downloads have moved to https://downloads.python.org/pypy/; some of the archives are no longer available.
Portable PyPy has moved from bitbucket.org to Github. Old archives have been moved to a 3rd-party "Bitbucket Archive" site.
* Update Stackless download links. Remove releases that are no longer available.
Stackless has moved from Bitbucket to Github. Old downloads have been renamed(?); stackless.com no longer works via HTTPS.
* Delete releases that have become invalid since the last check
* fix changed checksums
