*: Update PyPy-related links from bitbucket to heptapod

PyPy changed home from bitbucket to foss.heptapod recently[1]. The old
links on bitbucket no longer works. -> Update bitbucket links to their
correspondents on foss.heptapod.net .

[1] https://morepypy.blogspot.com/2020/02/pypy-and-cffi-have-moved-to-heptapod.html