logilab-common #1716128 umessage broken on python3 [validation pending]

test coverage is poor, so this only showed up with CubicWeb's tests.

prioritynormal
typebug
done in1.1.0
load1.000
load left0.000
closed by#6b64cb87416d [umessage] Use the same code paths for both python 2 and 3
patch[umessage] Use the same code paths for both python 2 and 3 [applied][umessage] import email exceptions from new module paths [applied][umessage] Work around bug in python 3's decode_header [applied]