So unless it a lot-lot of frames here's a not-python way.

• In preferences on the interface tab open the "Edit Shortcuts"
• Assign a not already used key combination to the command "Figure > Pose Symmetry > Swap Right and Left" (I used F12)
• Now Press f12, Right arrow, f12, right arrow, again ... 2 key strokes per frame.
• If you have a fancy programmable keyboard make a keyboard macro to just repeat as long as you hold some key down for a really lot of keys.

If it's less than maybe 100 frames this isn't too painful. Otherwise you'll probably need a Python solution. I'd be surprised if there wasn't already a script to do this.