If I were doing this, I'd use a latching single pole double throw relay. Your wall switches are single pole double throws switches, as well. Connecting the switch and relay in line like the below image will allow the wall switch or relay to toggle the light, regardless of each other's state. Using a latching relay allows you to turn on the relay with the raspberry pi's digital output, then stop driving it, and it will hold it's state.
I may be overstepping myself here, as this may not be the functionality you're looking for. The circuit I've shown below is similar to how a room with two light switches is wired. Hitting either switch, regardless of the other switch's position, turns the light on or off. This can be extended to any number of switches, as well, so you could, if you really wanted to, have 100 raspberry pi all switching the same light on and off.
simulate this circuit – Schematic created using CircuitLab
Note: Any values you see in the diagram are placeholders. You'll need to choose the correct components yourself.
Edit: Removed the resistor after actually looking at the device linked to. Added a flyback diode to the relay to protect against transient spikes when the relay is de-energized. Add a comment if you don't understand what this is for.
You'll need to check the datasheet to see what's going on with the relay you linked. Chances are, there's an onboard relay driver, in which case the flyback diode is probably already present. Adding it at your pi's output won't make a difference. Also, as noted in the comments, a latching relay usually has separate set and reset coils, which are not present in this schematic. In the end, though, that's not important, as the relay you linked is not a latching relay. Have fun driving that pin all day.