diskimage-builder (current) cannot build Centos6 images

Centos6 ships with python2.6.

Some elements (e.g. pkg-map) are using argparse - which was added with python 2.7.

Therefore the build fails with:

export DIB_RELEASE="6"
disk-image-create -x --image-size=5G -o target_centos-6 centos-minimal

++ find /tmp/in_target.d/pre-install.d -maxdepth 1 -name 'package-installs-*'
+ '[' -n '' ']'
+ package-installs-v2 --phase pre-install.d /tmp/package-installs.json
Traceback (most recent call last):
  File "/usr/local/bin/package-installs-v2", line 17, in <module>
    import argparse
ImportError: No module named argparse
++ /home/dib/devel/diskimage-builder/lib/img-functions:run_in_target:66 : check_break after-error run_in_target bash

Checked this on Debian-Jessie and Ubuntu-Xenial.

Centos6 is currently supported as build and target system [1].
Either the status of Centos6 has to be changed or all elements using argparse must be adapted.

I'm using diskimage-builder HEAD from today (commit 7603f97cadbf92c79549547dabbe252650343cac).

[1] doc/source/user_guide/supported_distros.rst