title: Issue using exponents in sympy
body: <p>I would like to setup an equation using sympy. But the use of exponents seems to be throwing an error:</p>
  
  <pre><code>import sympy
  from sympy import init_printing
  
  init_printing(use_unicode=True)
  
  
  E0, V0, B0, dB0, V  = sympy.symbols('E0 V0 B0 dB0 V')
  
  E0 + 9*V0*B0/16 * (
    [(V0/V)^2/3 - 1 ]^3 * dB0 + \
    [(V0/V)^2/3 - 1 ]^2 * [6-4*(V0/V)^(2/3)] \
  )
  </code></pre>
  
  <p>Returns the error:</p>
  
  <pre><code>TypeError: unsupported operand type(s) for ^: 'Mul' and 'int'
  </code></pre>