Just use annotate and specify axis coordinates. For example, “upper left” would be:
plt.annotate('Something', xy=(0.05, 0.95), xycoords="axes fraction")
You could also get fancier and specify a constant offset in points:
plt.annotate('Something', xy=(0, 1), xytext=(12, -12), va="top"
xycoords="axes fraction", textcoords="offset points")
For more explanation see the examples here and the more detailed examples here.