From a conversion through 2to3:

<       default_value=unicode("", "utf-8"),
---
>       default_value=str("", "utf-8"),

The Python 2 unicode constructor takes an optional second parameter which is the codec to use to convert when the first parameter is non-unicode.

2to3 should check the parameters on uses of unicode() and if there is a second parameter and the first is explicitly b"" bytes it should turn it into
  default_value=b"whatever".decode(second_param)

if the first is valid utf-8 and the second is "utf-8" (or its other spellings) it should leave it as is and simply become:
  default_value="thing passed to unicode() that was already utf-8"