[Build-common-hackers] Bug#529284:

[Olivier Aubert]

This bug could be closed, since DEB_PYTHON_SYSTEM is mentioned in the
current version of the docs. And then immediately reopened, since it
seems that recently, pysupport and pycentral have been deprecated in
favor of dh_python2/3. So DEB_PYTHON_SYSTEM is deprecated.

Transitioning from pysupport/pycentral should mainly consist in removing
the DEB_PYTHON_SYSTEM line. However, dh_python autodetection only works
for debian modules whose name is prefixed with python- or python3-,
following Debian Python Policy.

If the package contains an app (not a module) then the package name
should be explicitly listed in DEB_PYTHON2_MODULE_PACKAGES to be handled
by the python2 install helper, or in DEB_PYTHON3_MODULE_PACKAGES for 
Python3.