test: get rid of iteritems()

For Python 3, PEP 3106 changed the design of the dict builtin and the
mapping API in general to replace the separate list based and iterator
based APIs in Python 2 with a merged, memory efficient set and multiset
view based API. This new style of dict iteration was also added to the
Python 2.7 dict type as a new set of iteration methods. PEP-0469 [1]
recommends to replace d.iteritems() to iter(d.items()) to make code
compatible with Python 3.

1. https://www.python.org/dev/peps/pep-0469/

Part of #5538

test/app-tap/httpd.py

@@ -66,7 +66,7 @@ def read_handle(env, response):
     body = ["Not Found"]
     if env["PATH_INFO"] in paths:
         code, body, headers = paths[env["PATH_INFO"]]()
-    for key,value in env.iteritems():
+    for key,value in iter(env.items()):
         if "HTTP_" in key:
             headers.append((key[5:].lower(), value))
     response(code, headers)