Issue #17849: Raise sensible exception for invalid HTTP tunnel response

Initial patch from Cory Benfield.
diff --git a/Misc/NEWS b/Misc/NEWS
index 017acc6..26e41c3 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -37,6 +37,10 @@
 Library
 -------
 
+- Issue #17849: Raise a sensible exception if an invalid response is
+  received for a HTTP tunnel request, as seen with some servers that
+  do not support tunnelling.  Initial patch from Cory Benfield.
+
 - Issue #16180: Exit pdb if file has syntax error, instead of trapping user
   in an infinite loop.  Patch by Xavier de Gaye.