ascii pprint of ProductSet uses non-ascii multiplication symbol

[cbm755]

>>> srepr(A)
'ProductSet(FiniteSet(Rational(13, 111), Integer(2), Integer(3)), FiniteSet(exp(Integer(10)), Mul(Rational(1, 2), pi)))'

>>> pprint(A, use_unicode=True)
⎧ 13      ⎫   ⎧π   10⎫
⎨───, 2, 3⎬ × ⎨─, ℯ  ⎬
⎩111      ⎭   ⎩2     ⎭

>>> pprint(A, use_unicode=False)
  13           pi   10 
{---, 2, 3} × {--, e  }
 111           2       

Last one should be x not \xd7. See printing/pretty/pretty.py.

Possible fix might be:
prod_char = U('MULTIPLICATION SIGN') if self._use_unicode else 'x'

Also needs a test---can be simplier than mine.

[tokencolour]

I'll change the tests if they fail. Thanks, @cbm755 .