import matplotlib.pyplot as plt
import numpy as np

x, y = np.meshgrid(np.arange(0, 10), np.arange(0, 10))
z = np.max(np.dstack([abs(x), abs(y)]), 2)

fig, ax = plt.subplots(figsize=(3.5, 3), constrained_layout=True)
ax.contour(x, y, z, colors='0.50', linewidths=8.0, zorder=10) 
cs = ax.contour(x, y, z, zorder=11)
clabels = cs.clabel(colors='k', zorder=12)
# currently the zorder kwarg is not supported
# hence the following lines are required to set the zorder 
# for the contour labels' text manually.
# for clabel in clabels: 
#    clabel.set_zorder(12)
fig.savefig('clabel_zorder.png')