Basend on the example of Gary Kerr and as proposed here one may create empty circles related to specified values with following code:
import matplotlib.pyplot as plt
import numpy as np
from matplotlib.markers import MarkerStyle
x = np.random.randn(60)
y = np.random.randn(60)
z = np.random.randn(60)
g=plt.scatter(x, y, s=80, c=z)
g.set_facecolor('none')
plt.colorbar()
plt.show()