Use shallow copies instead of deepcopy in Learner2D.__getstate__

Key-level mutations (stack pops/inserts) are all that restore() needs to
roll back; values are never mutated in place. A deepcopy of the full
data dict would add O(npoints) allocation to every
ask(tell_pending=False) call and to pickling.

Author: basnijholt

adaptive/learner/learner2D.py

@@ -4,7 +4,7 @@
 import warnings
 from collections import OrderedDict
 from collections.abc import Callable, Iterable
-from copy import copy, deepcopy
+from copy import copy
 from math import sqrt
 
 import cloudpickle
@@ -897,8 +897,8 @@ def __getstate__(self):
             cloudpickle.dumps(self.function),
             self.bounds,
             self.loss_per_triangle,
-            deepcopy(self._stack),
-            deepcopy(self._get_data()),
+            self._stack.copy(),
+            self._get_data().copy(),
         )
 
     def __setstate__(self, state):