Gate python 2/3 shebang fix on version of pip used

[edalquist]

This allows a user to override duplicity_pip_pip_executable and get the correct shebang environment setup

[tersmitten]

I added this when statement because although 18.04 has python 3 duplicity is broken. We probably should do some more investigation. Or did you already do that?

[edalquist]

I'm actually using this role on Raspbian. The PPA repository for duplicity doesn't have arm binaries so I'm using the pip installer with pip3 explicitly named.

- name: "Install pip (rPi)"
  when: inventory_hostname in groups['rpi']
  become: true
  apt:
    update_cache: yes
    pkg:
      - python3-pip

- name: Install duplicity (pip)
  when: inventory_hostname in groups['rpi']
  include_role:
    name: oefenweb.duplicity
    apply:
      become: true
  vars:
    duplicity_install_method: pip
    duplicity_pip_pip_executable: pip3
    duplicity_pip_dependencies_python:
      - duplicity
      - paramiko

With this setup and the current logic the installed duplicity script fails to run as it was installed in using python3 but the fix shebang step essentially breaks things as it ends up trying to run it using python2