Building packages with python-3.4

[Leo Famulari <leo@famulari.name>]

On core-updates, we updated python to 3.5.2, but we kept 3.4.5 in case
we needed it.

I recently tried building python-cryptography with 3.4.5, like this:

    (arguments
     `(#:python ,python-3.4))

But, it fails like this:

patch-shebang: ./setup.py: warning: no binary for interpreter `python' found in $PATH
phase `patch-source-shebangs' succeeded after 0.2 seconds
starting phase `patch-generated-file-shebangs'
phase `patch-generated-file-shebangs' succeeded after 0.1 seconds
starting phase `build'
running "python setup.py" with command "build" and parameters ()
In execvp of python: No such file or directory
phase `build' failed after 0.0 seconds

I assume that python-wrapper needs to be adjusted somehow, but I'm not
sure. Any advice?