add a SETUP_WITH opcode

It speeds up the with statement and correctly looks up the special
methods involved.
diff --git a/Misc/NEWS b/Misc/NEWS
index 5526d9c..a6b5fce 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,9 @@
 Core and Builtins
 -----------------
 
+- Issue #6101: A new opcode, SETUP_WITH, has been added to speed up the with
+  statement and correctly lookup the __enter__ and __exit__ special methods.
+
 - Issue #5829: complex("1e500") no longer raises OverflowError.  This
   makes it consistent with float("1e500") and interpretation of real
   and imaginary literals.