[Python] Add deprecation warnings for the old Python module

The old Python module will be removed after releasing Cantera 2.1.

doc/sphinx/python/index.rst

@@ -4,6 +4,15 @@
 Python Module Documentation (Legacy)
 ====================================
 
+.. warning::
+
+   This version of the Cantera Python module is deprecated. The last version
+   of Cantera that this module will be included with is Cantera 2.1. Starting
+   with Cantera 2.1, a new Python module (based on the Cython package) is
+   available. When compiling Cantera, the new module can be build by using the
+   SCons option ``python_package=new``. Changes to the user interface are documented in :ref:`sec-cython-documentation`.
+
+
 Contents:
 
 .. toctree::

interfaces/python/Cantera/__init__.py

@@ -15,6 +15,13 @@ from importFromFile import *
 import os as _os
 import sys as _sys
 
+import warnings
+warnings.warn(
+    "\nThis version of the Cantera Python module is deprecated and will not be\n"
+    "available in Cantera 2.2 or later. For details on the new module, see\n"
+    "http://cantera.github.io/dev-docs/sphinx/html/cython/index.html\n",
+    stacklevel=2)
+
 if not os.getenv('PYTHON_CMD'):
     # Setting PYTHON_CMD here avoids issues with .cti -> .xml conversions
     # in cases where the python interpreter isn't in the system path.