title: AttributeError: 'datetime.timezone' object has no attribute '_utcoffset'
body: <p>I have lost ability to plot against datetime object after upgrading of matplotlib.</p>
  
  <p>Error is following:</p>
  
  <pre><code>  File "/opt/anaconda3/envs/py36/lib/python3.6/site-packages/matplotlib/pyplot.py", line 2813, in plot
      is not None else {}), **kwargs)
    File "/opt/anaconda3/envs/py36/lib/python3.6/site-packages/matplotlib/__init__.py", line 1810, in inner
      return func(ax, *args, **kwargs)
    File "/opt/anaconda3/envs/py36/lib/python3.6/site-packages/matplotlib/axes/_axes.py", line 1611, in plot
      for line in self._get_lines(*args, **kwargs):
    File "/opt/anaconda3/envs/py36/lib/python3.6/site-packages/matplotlib/axes/_base.py", line 393, in _grab_next_args
      yield from self._plot_args(this, kwargs)
    File "/opt/anaconda3/envs/py36/lib/python3.6/site-packages/matplotlib/axes/_base.py", line 370, in _plot_args
      x, y = self._xy_from_xy(x, y)
    File "/opt/anaconda3/envs/py36/lib/python3.6/site-packages/matplotlib/axes/_base.py", line 204, in _xy_from_xy
      bx = self.axes.xaxis.update_units(x)
    File "/opt/anaconda3/envs/py36/lib/python3.6/site-packages/matplotlib/axis.py", line 1475, in update_units
      self.set_units(default)
    File "/opt/anaconda3/envs/py36/lib/python3.6/site-packages/matplotlib/axis.py", line 1548, in set_units
      self._update_axisinfo()
    File "/opt/anaconda3/envs/py36/lib/python3.6/site-packages/matplotlib/axis.py", line 1490, in _update_axisinfo
      info = self.converter.axisinfo(self.units, self)
    File "/opt/anaconda3/envs/py36/lib/python3.6/site-packages/pandas/plotting/_converter.py", line 353, in axisinfo
      majfmt = PandasAutoDateFormatter(majloc, tz=tz)
    File "/opt/anaconda3/envs/py36/lib/python3.6/site-packages/pandas/plotting/_converter.py", line 367, in __init__
      self._tz._utcoffset = self._tz.utcoffset(None)
  AttributeError: 'datetime.timezone' object has no attribute '_utcoffset'
  </code></pre>
  
  <p>Is it possible to upgrade without loosing capabilities in Python?</p>