cmd.py: importing matplotlib without X can raise RuntimeErrors.

cmd.py: importing matplotlib without X can raise RuntimeErrors.

For example, with the GTKAgg backend selected in
~/.matplotlib/matplotlibrc, running cmd.py without X raises:

  Traceback (most recent call last):
    File "./cmd.py", line 32, in <module>
      from matplotlib import pyplot as _pyplot
    ...
  RuntimeError: could not create GdkCursor object