Series around Inf produces series around -Inf

Hi, I'm getting the following behaviour:

```
julia> using SymPy

julia> @syms k
(k,)

julia> series(atan(k), x=k, x0=Inf)
1 1 1 π ⎛1 ⎞
- ──── + ──── - ─ - ─ + O⎜──; k → -∞⎟
5 3 k 2 ⎜ 6 ⎟
5⋅k 3⋅k ⎝k ⎠
```

With `x0=-Inf`, I get the same (this time correct) result. Running the same in python works fine.