[Python] How to find the most common frequeny in Time series

I have a TimeSeries shown below. We can clearly see that there is a cyclical behavior in the data, where rise and fall happens every 12 months and so on, irrespective of if trend is rising/falling. I am struggling to find how to extract this frequency of 12 from the data using scipy.fft python library. Mostly every time repetition happens around 12 months, but it can be around 11 months as well or 13 months, but 12 is the most common frequency. I am using using Fourier transformation, but can't figure out how to extract this most common frequency (12) using some python library?

Thanks



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