py3: fix generated non-ascii message in test-notify.t
The resulting "Subject:" header contains an encoded word in Python 3 so
we have to add distinct outputs for Python 3 but underlying values are
the same:
>>> from email.header import decode_header
>>> decode_header('=?utf-8?b?w6AuLi4=?=')
[(b'\xc3\xa0...', 'utf-8')]
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