This versioning scheme is consistent with other
tools in the python ecosystem so it feels more natural
and allows better interaction with other tools.
fixes#346
`pypyX.Y.exe` executables are missing from PyPy archives on Windows before v7.3.9 (X.Y < 3.9)
`pypy2.7` symlinks are also missing from macOS/Linux PyPy archives before v7.3.9
relates to #346