refactor(bin/python_lib): move six.py under bin/python_lib

Following @favonia's suggestion in #393

bin/.gitignore

@@ -1,3 +1,4 @@
 lean
 lean.exe
-six.pyc
+python_lib/six.pyc
+python_lib/__init__.pyc

bin/linja

@@ -9,8 +9,8 @@
 
 # Python 2/3 compatibility
 from __future__ import print_function
-import six
-from six.moves import filter, map
+import python_lib.six as six
+from python_lib.six.moves import filter, map
 
 import argparse
 import fnmatch
@@ -25,7 +25,7 @@ import subprocess
 import sys
 import tempfile
 import threading
-from six.moves import urllib
+from python_lib.six.moves import urllib
 
 # Enforce subprocesses to use 'utf-8' in Python 2
 if six.PY2: