Unknown projection '3d' in multistage_drone_race.py example

[ivrolan]

When executing multistage_drone_race.py with matplotlib version 3.1.2, a Value error arises:

See Traceback

      Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/matplotlib/projections/__init__.py", line 58, in get_projection_class
    return projection_registry.get_projection_class(projection)
  File "/usr/lib/python3/dist-packages/matplotlib/projections/__init__.py", line 25, in get_projection_class
    return self._all_projection_types[name]
KeyError: '3d'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "multistage_drone_race.py", line 352, in <module>
    ax = fig.gca(projection='3d')
  File "/usr/lib/python3/dist-packages/matplotlib/figure.py", line 1932, in gca
    return self.add_subplot(1, 1, 1, **kwargs)
  File "/usr/lib/python3/dist-packages/matplotlib/figure.py", line 1396, in add_subplot
    self._process_projection_requirements(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/matplotlib/figure.py", line 1120, in _process_projection_requirements
    projection_class = projections.get_projection_class(projection)
  File "/usr/lib/python3/dist-packages/matplotlib/projections/__init__.py", line 60, in get_projection_class
    raise ValueError("Unknown projection %r" % projection)
ValueError: Unknown projection '3d'

I fixed this issue in the pull-request I will open by adding this line to the example as in this StackOverflow Question:

from mpl_toolkits.mplot3d import Axes3D