py3: fix test-diff-newlines.t to be compatible with py3

contrib/python3-whitelist

@@ -11,6 +11,7 b' test-contrib-check-code.t'
 test-contrib-check-commit.t
 test-ctxmanager.py
 test-diff-issue2761.t
+test-diff-newlines.t
 test-diff-reverse.t
 test-diff-subdir.t
 test-dirstate-nonnormalset.t

tests/test-diff-newlines.t

@@ -1,6 +1,6 b''
   $ hg init
 
-  $ $PYTHON -c 'file("a", "wb").write("confuse str.splitlines\nembedded\rnewline\n")'
+  $ $PYTHON -c 'open("a", "wb").write(b"confuse str.splitlines\nembedded\rnewline\n")'
   $ hg ci -Ama -d '1 0'
   adding a