py3: use open() instead of file()

file() is not present in Python 3

Differential Revision: https://phab.mercurial-scm.org/D2044

hgext/mq.py

@@ -281,7 +281,7 b' class patchheader(object):'
         nodeid = None
         diffstart = 0
 
-        for line in file(pf):
+        for line in open(pf, 'rb'):
             line = line.rstrip()
             if (line.startswith('diff --git')
                 or (diffstart and line.startswith('+++ '))):