When building relayout/restyle strings don't use brackets

'foo[0].bar' -> 'foo.0.bar'

For unknown reasons, this caused a jitter in the datashader example

Author: Jon M. Mease

Diff for: plotly/basedatatypes.py

@@ -2733,7 +2733,7 @@ def _prop_set_child(self, child, prop_path_str, val):
         child_prop_val = getattr(self, child.plotly_name)
         if isinstance(child_prop_val, (list, tuple)):
             child_ind = BaseFigure._index_is(child_prop_val, child)
-            obj_path = '{child_name}[{child_ind}].{prop}'.format(
+            obj_path = '{child_name}.{child_ind}.{prop}'.format(
                 child_name=child.plotly_name,
                 child_ind=child_ind,
                 prop=prop_path_str)