Python plot circunference arc with a tangent line from external point P

I need to drawing in Python an arc and the line tangent to it from an external point P, so find line equation and the tangent point T coordinates (xt,yt)

I want solve it using libraries Matplotlib, numpy.

I have those avaiable datas:
- Circunference equation: x^2 + y^2 - 2ay + b = 0 ;
(a, b are parameter well know so also its center is defined O(x0y0))
- External point coordinates: P : (x1,y1)

I posted an example of what i m asking.