Python 3 compatibility

When installing env through pip on Python 3, calls to `prefix` fail at a `print` statement using the Python 2 syntax. However, that statement isn't in the source in this repo.

Reproduce the failure when installing env through pip on Python 3 and calling prefix. Search the repository for prefix and Python 2-style print statements, since the reported statement is not in the visible source. Done means prefix installs and runs successfully under Python 3.

Written by the indexing model from the issue text.