Do not modify DataObjectHandleBase python binding on add
This fixes the unexpected behaviour
from GaudiKernel.DataObjectHandleBase import DataObjectHandleBase
a = DataObjectHandleBase('a:b:c')
print a
# DataObjectHandleBase("a:b:c")
print a + '1'
# DataObjectHandleBase("a1:b1:c1")
print a
# DataObjectHandleBase("a1:b1:c1")