We must coerce name strings with `tounicode` because, on Python 2, the ElementTree library that we use to parse the designspace's XML will return ASCII-encoded `bytes` (PY2 `str`) whenever it can; and only returns `unicode` (PY3 `str`) when strings can't be encoded as ASCII... :(
However the `addName` method requires unicode strings (it wouldn't be able to encode them with multiple platform-specific encodings otherwise).
So we decode bytes as ASCII before passing them to addName.
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