py3: replace file() with open() in test-patch-offset.t

file() is not present in Python 3.

Differential Revision: https://phab.mercurial-scm.org/D2119

tests/test-patch-offset.t

@@ -5,7 +5,7 b''
   > path = sys.argv[1]
   > patterns = sys.argv[2:]
   > 
-  > fp = file(path, 'wb')
+  > fp = open(path, 'wb')
   > for pattern in patterns:
   >     count = int(pattern[0:-1])
   >     char = pattern[-1] + '\n'