Update CONTRIBUTING.rst

Author: chalmerlowe

CONTRIBUTING.rst

@@ -22,7 +22,7 @@ In order to add a feature:
   documentation.
 
 - The feature must work fully on the following CPython versions:
-  3.8, 3.9, 3.10, 3.11, 3.12 and 3.13 on both UNIX and Windows.
+  3.8, 3.9, 3.10, 3.11, 3.12, and 3.13 on both UNIX and Windows.
 
 - The feature must not add unnecessary dependencies (where
   "unnecessary" is of course subjective, but new dependencies should