REF: Fix multi-scale DatetimeTickFormatter deprecation in Bokeh 3.0

> BokehDeprecationWarning: Passing lists of formats for DatetimeTickFormatter scales was deprecated in Bokeh 3.0. Configure a single string format for each scale

Fixes #807

Author: kernc

backtesting/_plotting.py

@@ -247,8 +247,8 @@ def plot(*, results: pd.Series,
     if is_datetime_index:
         fig_ohlc.xaxis.formatter = CustomJSTickFormatter(
             args=dict(axis=fig_ohlc.xaxis[0],
-                      formatter=DatetimeTickFormatter(days=['%d %b', '%a %d'],
-                                                      months=['%m/%Y', "%b'%y"]),
+                      formatter=DatetimeTickFormatter(days='%a, %d %b',
+                                                      months='%m/%Y'),
                       source=source),
             code='''
 this.labels = this.labels || formatter.doFormat(ticks