I saw the same failure on Ubuntu 20. Updating pyenv ($ pyenv update) solved the problem. Details follow.
During the 7.4 release, a .python-version file specifying pypy3 was added to the bridgepoint repository. I was building master, which contained this file, and the version of pyenv installed on my machine predated the version that supports fuzzy matching of python versions, so even though pypy3.9-7.3.9 was installed on my machine, pyenv would report that pypy3 was not installed.
Pyenv 2.3.6 includes the ability for pyenv to recognize that a requirement for pypy3 can be satisfied by any pypy3.* version. I was running pyenv 2.3.0, which does not have this feature. Updating pyenv solved the problem.