Fix plot issue after updated to matplotlib 3.7.0

[iaalm]

locator.set_view_interval is deprecated and replace with locator.axis.set_view_interval. The plot feature will break if updated to matplotlib 3.7.0.

[iaalm]

it breaks like

    cerebro.plot()
  File "/Users/simon/Projects/pretzel/py311/lib/python3.11/site-packages/backtrader/cerebro.py", line 989, in plot
    rfig = plotter.plot(strat, figid=si * 100,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/simon/Projects/pretzel/py311/lib/python3.11/site-packages/backtrader/plot/plot.py", line 262, in plot
    self.mpyplot.setp(ax.get_xticklabels(), visible=False)
                      ^^^^^^^^^^^^^^^^^^^^
  File "/Users/simon/Projects/pretzel/py311/lib/python3.11/site-packages/matplotlib/axes/_base.py", line 74, in wrapper
    return get_method(self)(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/simon/Projects/pretzel/py311/lib/python3.11/site-packages/matplotlib/axis.py", line 1451, in get_ticklabels
    return self.get_majorticklabels()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/simon/Projects/pretzel/py311/lib/python3.11/site-packages/matplotlib/axis.py", line 1408, in get_majorticklabels
    self._update_ticks()
  File "/Users/simon/Projects/pretzel/py311/lib/python3.11/site-packages/matplotlib/axis.py", line 1257, in _update_ticks
    major_locs = self.get_majorticklocs()
                 ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/simon/Projects/pretzel/py311/lib/python3.11/site-packages/matplotlib/axis.py", line 1479, in get_majorticklocs
    return self.major.locator()
           ^^^^^^^^^^^^^^^^^^^^
  File "/Users/simon/Projects/pretzel/py311/lib/python3.11/site-packages/matplotlib/dates.py", line 1378, in __call__
    locator = self.get_locator(dmin, dmax)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/simon/Projects/pretzel/py311/lib/python3.11/site-packages/backtrader/plot/locator.py", line 226, in get_locator
    locator.set_view_interval(*self.axis.get_view_interval())
    ^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'RRuleLocator' object has no attribute 'set_view_interval'

[iaalm]

@neilsmurphy could you help review this?

[iaalm]

Root cause should be matplotlib/matplotlib@21e5876