Issue #15225: improve error message when hmac is passed a wrong key type.
Patch by Marc Abramowitz.
diff --git a/Lib/hmac.py b/Lib/hmac.py
index 9c08023..4297a71 100644
--- a/Lib/hmac.py
+++ b/Lib/hmac.py
@@ -35,7 +35,7 @@
         """
 
         if not isinstance(key, bytes):
-            raise TypeError("expected bytes, but got %r" % type(key).__name__)
+            raise TypeError("key: expected bytes, but got %r" % type(key).__name__)
 
         if digestmod is None:
             import hashlib