Those are quote codes that Go strconv.Quote might produce. And even
though Python does not use them when quoting, it too handles those quote
codes when decoding:

    In [1]: '\r'
    Out[1]: '\r'

    In [2]: '\a\b\v\f'
    Out[2]: '\x07\x08\x0b\x0c'

https://github.com/python/cpython/blob/2.7.18-0-g8d21aa21f2c/Objects/stringobject.c#L677-L688

-> Teach strconv.unquote + friends to handle them as well.

/reviewed-by @jerome
/reviewed-on !14