Python 3: Fix parsing of received notification
Python 3: Fix parsing of received notification

The parsing of incoming notifications is performed with a helper
which normally resolves to a mimetools class.

For Python3, package mimetools doesn't exist anymore, and the helper
resolves to a class with incompatible signature.

In order to mimick the original helper behaviour and return a message
object as expected, use a different facility from package email.

Change-Id: I92f10755f1a98239df3b3a1494207299a3681a84
Closes-Bug: #1818046
Co-Authored-By: Michele Baldessari <47483730395cccb6448ad7313d5cedb48de25e40@acksyn.org>
Co-Authored-By: Alex Schultz <6963656cd5f1036a86f38d6c01e67c3f63074c58@redhat.com>

openstack/zaqar,openstack/zaqar,openstack/zaqar,openstack/zaqar