Estimate the angle in degrees between two photos

I would like to estimate the rotation in degrees between two images.

To explain myself better, I created Example 1 simple design with only one object.
In this example I would like python to return me 90 degrees of rotation.

A real scenario is instead Example 2 taken from stackoverflow
In this example, I would like to get those 9 degrees of rotation.

Can you help me?
For now I have only managed to make a FLANN based Matcher

thank you.

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