How simplistic an answer do you need? (apologies if this way underestimates where your question is coming from...)
Acceleration is change in speed[1] over time. Delta V over delta T. How many meters per second did you change your speed by every second?
So for a first approximation: Take three consecutive datapoints (a, b, and c). Calculate the distance and time between point a and b (from their lat/longs and timestamps). Divide the distance by the time to get average speed between a and b. Now do the same for points b and c. Now you can subtract one from the other to get "change in speed", and you can approximate the corresponding "change in time" several ways depending on the nature of your data (if you've got reliable one-point-per-second datapoints, just use 1 second as your "change in time". if your gps data isn't that consistent, I'd first try averaging the two time intervals a-b and b-c and using that.)
Now just keep doing that iteratively for every point and the two points following it to get a stream of acceleration datapoints and their relative timestamps. Turning this into Python is left as an exercise for the reader :-)
Edit: I just re-read the full question after posting this - if you've got reliable speeds for each timestamped datapoint, ignore the first half of that, all you need to to is calculate "the speed at time b minus speed at time a divided by time difference between time a and b" , then assign that as the acceleration for time b (for simplicity, or for some new average time halfway between time a and b to bet closer to "the truth"). That'll probably get you "close enough".
Note that if you're getting typical GPS data "noise", you'll probably want to track a moving average of the calculated accelerations. The multi rotor controller guys seem to mostly use "kalman filters" to clean up and make sense out of real world noisy sensor data... For your use case of algorithmicly generating timestamps in video, you quite likely wont need to get their level of precision. Try the easy way first...
[1] I'll handwave away the technicalities of the difference between speed and velocity for at least the first approximation solution here...