fix: python 2 support

Author: henryiii

src/boost_histogram/_internal/axis.py

@@ -10,6 +10,7 @@
 from .six import string_types
 
 import copy
+
 from typing import Dict, Any, TYPE_CHECKING
 
 del absolute_import, division, print_function
@@ -53,6 +54,12 @@ def __copy__(self):
         other._ax = copy.copy(self._ax)
         return other
 
+    def __setstate__(self, state):
+        self._ax = state["_ax"]
+
+    def __getstate__(self):
+        return {"_ax": self._ax}
+
     def __getattr__(self, item):
         if item == "_ax":
             return Axis.__dict__[item].__get__(self)