Usually, constructors try to accept format returned by repr(obj), or even str(obj). It's the case for Fraction:

>>> str(fractions.Fraction(1, 2))
'1/2'
>>> fractions.Fraction("1/2")
Fraction(1, 2)

It works as expected.

I dislike the idea of trying to handle more Unicode characters which "look like" "/", or characters like "⅔". It sounds like a can of worm, and I don't think that such feature belongs to the stdlib. You can easily write your helper function accepting string and returning a fraction.

If someone is motivated to accept more character, I would prefer to have an unified proposition covering all Python number types (int, float, Fraction, complex, etc.) and listing all characters. Maybe a PEP would make sense.