Remove warning from mpl 3.10

Author: cphyc

labellines/core.py

@@ -238,7 +238,7 @@ def labelLines(
             xvals[i] = new_xv  # type: ignore
 
     # Convert float values back to datetime in case of datetime axis
-    if isinstance(ax.xaxis.converter, DateConverter):
+    if isinstance(ax.xaxis.get_converter(), DateConverter):
         xvals = [
             num2date(x).replace(tzinfo=ax.xaxis.get_units())
             for x in xvals  # type: ignore