GNUmed <-> Python 3

GNUmed is currently running on Python 2 but it will
eventually have to be migrated to Python 3. This will,
however, not happen before Debian acquires Python 3 packages
for wxPython.

The recommended way to migrate is to first make code
compatible with running on both Python 2 and Python 3
which effort I have now started. It is most easily done
when restricting Python 2 use to version 2.7.

To that end, starting with version 1.6 GNUmed will
require users to run Python 2.7 (which will not be EOLed
before 2020). The next 1.5 release will add a reminder
to upgrade if need be.

Karsten
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