Fix #3669: rsolve_hyper can't find solution for F(n) + F(n - 2) + F(n - 1)==0

This time it fails, as the characteristic equation for this recurrence relation has imaginary roots:
```
In [1]: rsolve_hyper([1,1,1], 0, n)
Out[1]: 
                  n                     n
   ⎛        ___  ⎞       ⎛        ___  ⎞ 
   ⎜  1   ╲╱ 3 ⋅ⅈ⎟       ⎜  1   ╲╱ 3 ⋅ⅈ⎟ 
C₀⋅⎜- ─ - ───────⎟  + C₁⋅⎜- ─ + ───────⎟ 
   ⎝  2      2   ⎠       ⎝  2      2   ⎠ 

```