Free image memory in Python darknet.py

[betegon]

Hi @AlexeyAB @cenit @WongKinYiu

Following the pull request #6668 , It is a good practice to move the free_image() call inside detect_image() function in darknet.py. This change is suggested as it could lead to errors, as for now you have to call detect_image() and then free_image() in order to detect and image.

Given way I propose here, users only have to call detect_image() to detect and image, reducing code duplication.

This code has been tested and it works perfectly (it has been running detections for several hours without increasing memory usage)