py3: replace file() with open() in test-mq-missingfiles.t

file() is not present in Python 3.

Differential Revision: https://phab.mercurial-scm.org/D2121

tests/test-mq-missingfiles.t

@@ -9,7 +9,7 b' future qrefresh.'
   > args = sys.argv[2:]
   > assert (len(args) % 2) == 0
   > 
-  > f = file(path, 'wb')
+  > f = open(path, 'wb')
   > for i in xrange(len(args)/2):
   >    count, s = args[2*i:2*i+2]
   >    count = int(count)