sympy/sympy — pull request #1987 by smichr (2013-04-09 00:24:17+00:00)
Title: -1 and I flatten better in Mul.flatten

This addresses issue https://code.google.com/p/sympy/issues/detail?id=3737

Previously
>>> (-1)**Rational(1,3)*I
(-1)**(1/3)*I

Now
>>> powsimp(_)
(-1)**(5/6)