backport hmac.compare_digest to partially implement PEP 466 (closes #21306)

Backport from Alex Gaynor.
diff --git a/Lib/hmac.py b/Lib/hmac.py
index 5388106..9cd1a9f 100644
--- a/Lib/hmac.py
+++ b/Lib/hmac.py
@@ -5,6 +5,9 @@
 
 import warnings as _warnings
 
+from operator import _compare_digest as compare_digest
+
+
 trans_5C = "".join ([chr (x ^ 0x5C) for x in xrange(256)])
 trans_36 = "".join ([chr (x ^ 0x36) for x in xrange(256)])