strconv: Fix unquote to handle \a \b \v \f

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