Fix specgram test on NumPy 1.20.

In NumPy 1.20, the fft is slight different (more accurate?) and returns
0 as the last entry, instead of some small number ~1e-30. This causes a
divide-by-zero warning in `log10`. However, this test doesn't really
care about the spectrogram, just the frequencies, so we can change the
scale to linear to avoid the warning.

Author: QuLogic

lib/matplotlib/tests/test_axes.py

@@ -4452,7 +4452,7 @@ def test_specgram_angle():
 
 def test_specgram_fs_none():
     """Test axes.specgram when Fs is None, should not throw error."""
-    spec, freqs, t, im = plt.specgram(np.ones(300), Fs=None)
+    spec, freqs, t, im = plt.specgram(np.ones(300), Fs=None, scale='linear')
     xmin, xmax, freq0, freq1 = im.get_extent()
     assert xmin == 32 and xmax == 96