Pipfiles can specify which Python version they are compatible with. But that is different from what pyenv does; pyenv is the tool that can install different python binaries on your computer.
For example if you are running Python 3.7 and you have a Pipfile that says it requires Python 3.5
python_version = '3.5'
you can use pyenv to install Python 3.5. After Python 3.5 is installed, Pipenv can then install the application’s dependencies (using Python 3.5) and run it.