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

file() is not present in Python 3.
This patch also adds a b'' prefix to make sure we write bytes in Python 3.

Differential Revision: https://phab.mercurial-scm.org/D2118

tests/test-relink.t

@@ -49,7 +49,7 b" don't sit forever trying to double-lock "
 
 Test files are read in binary mode
 
-  $ $PYTHON -c "file('.hg/store/data/dummy.i', 'wb').write('a\r\nb\n')"
+  $ $PYTHON -c "open('.hg/store/data/dummy.i', 'wb').write(b'a\r\nb\n')"
   $ cd ..
 
 
@@ -68,7 +68,7 b' clone and pull to break links'
   $ echo b >> b
   $ hg ci -m changeb
   created new head
-  $ $PYTHON -c "file('.hg/store/data/dummy.i', 'wb').write('a\nb\r\n')"
+  $ $PYTHON -c "open('.hg/store/data/dummy.i', 'wb').write(b'a\nb\r\n')"
 
 
 relink