The `${prefix}/bin/pyvenv` will not exist if the current version is not a Python 3.3+. IMHO, checking the existence of `pyvenv` executable as same manner as `pyenv-virtualenv` might be better here.

https://github.com/yyuu/pyenv-virtualenv/blob/579f4784c839e1daf3302a1f1d6f058c9a4fd8b7/bin/pyenv-virtualenv#L134