BUG: clabel manual mode doesn't make a gap big enough in contour

Bug summary

Using plt.clabel(manual=True) adds labels but the labels are not easily readable because only a small gap is created in the contour.

Code for reproduction

The following works:

plt.figure()
x = np.arange(400).reshape((20,20))
c = plt.contour(x)
plt.clabel(c)

But the following doesn't give as nice a result:

plt.figure()
x = np.arange(400).reshape((20,20))
c = plt.contour(x)
plt.clabel(c, manual=True)
# click around

This happens with both the Qt5 and MacOS X backends, and I am using a retina display.

Matplotlib version

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