_DummyAxis needs a __name__ attribute for ScalarFormatter

[efiring]

Bug report

Bug summary

ScalarFormatter.format_data_short(x) uses self.axis__name__ to get xaxis or yaxis info. If the instance was made with a _DummyAxis instance, however, it fails.
Code for reproduction
See next comment.
Matplotlib version
Master (3.2.1.post2853+g15c0d7c39)

[efiring]

This bug is triggered in normal usage by the following, if you move the cursor over the image:

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.colors as mcolors

z = np.arange(9).reshape((3, 3)).astype(float)
fig, ax = plt.subplots()
im = ax.imshow(z)
cbar = fig.colorbar(im, ax=ax, boundaries=[2, 4, 6])
plt.show()

In this case, the boundaries kwarg is causing the use of a _DummyAxis, which lacks a __name__ attribute.
The problem was noticed while looking at #17453.