ui: use bytes IO and convert EOL manually in ui.editor()
Text IO sucks on Python 3 as it must be a unicode stream. We could introduce
a wrapper that converts unicode back to bytes, but it wouldn't be simple to
handle offsets transparently from/to underlying IOBase API.
Fortunately, we don't need to process huge text files, so let's stick to
bytes IO and convert EOL in memory.
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