Update sys.path example in INSTALL.

* INSTALL: The example for checking sys.path was syntactically invalid
for Python 3.x, so it has been fixed to work there as well as relatively
modern 2.x (tested with 2.5 since that's the oldest I had on hand).

INSTALL

@@ -47,7 +47,7 @@ your path (/usr/local/bin/ or ~/bin/ for example). Similarly, weather.py
 needs to be somewhere in Python's include path. You can see your Python
 interpreter's default include path by running::
 
-   python -c "import sys ; print sys.path"
+   python -c 'import sys ; print(sys.path)'
 
 If the correlation data files are to be used (airports, places,
 stations, zctas, zones), they need to be in your current working