As of matplotlib 3.3.0, Axes3D.set_box_aspect seems to be the recommended approach.
import numpy as np
import matplotlib.pyplot as plt
xs, ys, zs = ...
ax = plt.axes(projection='3d')
ax.set_box_aspect((np.ptp(xs), np.ptp(ys), np.ptp(zs))) # aspect ratio is 1:1:1 in data space
ax.plot(xs, ys, zs)