Fix file closing in test_http_cookiejar.

Lib/test/test_http_cookiejar.py

@@ -263,7 +263,7 @@ class FileCookieJarTests(unittest.TestCase):
         # Invalid contents of cookies file (eg. bad magic string)
         # causes a LoadError.
         try:
-            f = open(filename, "w")
+            with open(filename, "w") as f:
                 f.write("oops\n")
                 for cookiejar_class in LWPCookieJar, MozillaCookieJar:
                     c = cookiejar_class()