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

file() is not present in Python 3.

Differential Revision: https://phab.mercurial-scm.org/D2134

tests/test-contrib.t

@@ -201,7 +201,7 b' too many labels'
 
 binary file
 
-  $ $PYTHON -c "f = file('binary-local', 'w'); f.write('\x00'); f.close()"
+  $ $PYTHON -c "f = open('binary-local', 'w'); f.write('\x00'); f.close()"
   $ cat orig >> binary-local
   $ $PYTHON simplemerge -p binary-local base other
   warning: binary-local looks like a binary file.