Switch deepcopy memo check to d in memo

pochmann benchmarked this issue's candidate fixes on gh-154594 and found
the try/except form is ~3.5x slower on a memo miss (the common case in
real deepcopy workloads) despite being fastest on a hit. `d in memo` ties
or beats the current `is not None` check on both hit and miss, and
matches where the competing PR (gh-154595) landed after the same
discussion.

Author: sreehariannam

Lib/copy.py

@@ -121,11 +121,8 @@ def deepcopy(x, memo=None):
     d = id(x)
     if memo is None:
         memo = {}
-    else:
-        try:
-            return memo[d]
-        except KeyError:
-            pass
+    elif d in memo:
+        return memo[d]
 
     copier = _deepcopy_dispatch.get(cls)
     if copier is not None: