fix side-effect of annotate that modifies the input dictionary

Author: leejjoon

lib/matplotlib/text.py

@@ -1799,7 +1799,7 @@ def __init__(self, s, xy,
         self.arrow = None
 
         if arrowprops and arrowprops.has_key("arrowstyle"):
-
+            arrowprops = self.arrowprops.copy()
             self._arrow_relpos = arrowprops.pop("relpos", (0.5, 0.5))
             self.arrow_patch = FancyArrowPatch((0, 0), (1,1),
                                                **arrowprops)