py3: replace file() with open() in test-mactext.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/D2122

tests/test-mactext.t

@@ -3,9 +3,9 b''
   > import sys
   > 
   > for path in sys.argv[1:]:
-  >     data = file(path, 'rb').read()
-  >     data = data.replace('\n', '\r')
-  >     file(path, 'wb').write(data)
+  >     data = open(path, 'rb').read()
+  >     data = data.replace(b'\n', b'\r')
+  >     open(path, 'wb').write(data)
   > EOF
   $ cat > print.py <<EOF
   > import sys