radius = 3
x,y = np.ogrid[-3:3:5j, -3:3:5j]
dist = ((x**2+y**2)**0.5)
mask = dist-radius).clip(0,None)
print(mask)

array([[1.24264069, 0.35410197, 0.        , 0.35410197, 1.24264069],
       [0.35410197, 0.        , 0.        , 0.        , 0.35410197],
       [0.        , 0.        , 0.        , 0.        , 0.        ],
       [0.35410197, 0.        , 0.        , 0.        , 0.35410197],
       [1.24264069, 0.35410197, 0.        , 0.35410197, 1.24264069]])

mask = mask/np.max(mask)
mask = 1 - mask
print(mask)

array([[0.        , 0.71504074, 1.        , 0.71504074, 0.        ],
       [0.71504074, 1.        , 1.        , 1.        , 0.71504074],
       [1.        , 1.        , 1.        , 1.        , 1.        ],
       [0.71504074, 1.        , 1.        , 1.        , 0.71504074],
       [0.        , 0.71504074, 1.        , 0.71504074, 0.        ]])