Update risch.py

Earlier
>>risch_integrate(sin(x),x)
raised NonImplementedError

After the above changes
>>risch_integrate(sin(x),x)
   ⅈ⋅x    -ⅈ⋅x
  ℯ      ℯ    
- ──── - ─────
   2       2  
<Now risch_integrate works for trigonometric, inverse trigonometric and hyperbolic functions>