py3: replace file() with open() in test-convert-p4-filetypes.t

file() is not present in Python 3. While I was here, I added b'' prefix to make
sure we write things correctly in Python 3.

Differential Revision: https://phab.mercurial-scm.org/D2112

tests/test-convert-p4-filetypes.t

@@ -52,7 +52,7 b' not testing these'
   >          p4 add -t $T file_$T2
   >          ;;
   >       binary*)
-  >          $PYTHON -c "file('file_$T2', 'wb').write('this is $T')"
+  >          $PYTHON -c "open('file_$T2', 'wb').write(b'this is $T')"
   >          p4 add -t $T file_$T2
   >          ;;
   >       *)