AttributeError: module 'matplotlib' has no attribute 'pyplot'

While attempting to create a piano roll with librosa and matplotlib, like this:

```
def plot_piano_roll(pm, start_pitch, end_pitch, fs=100):
# Use librosa's specshow function for displaying the piano roll
librosa.display.specshow(pm.get_piano_roll(fs)[start_pitch:end_pitch],
hop_length=1, sr=fs, x_axis='time', y_axis='cqt_note',
fmin=pretty_midi.note_number_to_hz(start_pitch))

plt.figure(figsize=(8, 4))
plot_piano_roll(pm, 56, 70)
```

I encounter this error:

```
AttributeError Traceback (most recent call last)
File ~/anaconda3/lib/python3.9/site-packages/matplotlib_inline/backend_inline.py:100, in show(close, block)
97 # only call close('all') if any to close
98 # close triggers gc.collect, which can be slow
99 if close and Gcf.get_all_fig_managers():
--> 100 matplotlib.pyplot.close('all')

File ~/anaconda3/lib/python3.9/site-packages/matplotlib/_api/__init__.py:226, in caching_module_getattr..__getattr__(name)
224 if name in props:
225 return props[name].__get__(instance)
--> 226 raise AttributeError(
227 f"module {cls.__module__!r} has no attribute {name!r}")

AttributeError: module 'matplotlib' has no attribute 'pyplot'
```

The error seems to be fixed if I explicitly declare `import matplotlib.pyplot as plt` at the start of the file and then use that to call it like this: `plt.close('all')`. If that alone is satisfactory, I would request to do a PR.