Skip Time.TimePlotDate doc-string example that imports matplotlib

Running `astropy.test()` fails when one does not have `matplotlib` installed because of a doc-string example in `TimePlotDate`:
```
      >>> import matplotlib.pyplot as plt
      >>> jyear = np.linspace(2000, 2001, 20)
      >>> t = Time(jyear, format='jyear', scale='utc')
      >>> plt.plot_date(t.plot_date, jyear)
      >>> plt.gcf().autofmt_xdate()  # orient date labels at a slant
      >>> plt.draw()
```
I think tests should only rely on numpy being present, so this PR skips the test by changing all `>>>` to `>>`. If there is a more elegant way, let me know.