Add python 3.12 to contributing.rst and constraints directory

CONTRIBUTING.rst

@@ -24,7 +24,7 @@ In order to add a feature to ``python-ndb``:
   documentation (in ``docs/``).
 
 - The feature must work fully on the following CPython versions:
-  3.7, 3.8, 3.9, 3.10, and 3.11 on both UNIX and Windows.
+  3.7, 3.8, 3.9, 3.10, 3.11 and 3.12 on both UNIX and Windows.
 
 - The feature must not add unnecessary dependencies (where
   "unnecessary" is of course subjective, but new dependencies should
@@ -260,12 +260,14 @@ We support:
 -  `Python 3.9`_
 -  `Python 3.10`_
 -  `Python 3.11`_
+-  `Python 3.12`_
 
 .. _Python 3.7: https://docs.python.org/3.7/
 .. _Python 3.8: https://docs.python.org/3.8/
 .. _Python 3.9: https://docs.python.org/3.9/
 .. _Python 3.10: https://docs.python.org/3.10/
 .. _Python 3.11: https://docs.python.org/3.11/
+.. _Python 3.12: https://docs.python.org/3.12/
 
 
 Supported versions can be found in our ``noxfile.py`` `config`_.