Mark v2.18.0 in doc/whatsnew

Author: khaeru

doc/whatsnew.rst

@@ -3,8 +3,11 @@
 What's new?
 ***********
 
-Next release
-============
+.. Next release
+.. ============
+
+v2.18.0 (2024-10-15)
+====================
 
 - Python 3.13 (`released 2024-10-07 <https://www.python.org/downloads/release/python-3130/>`_) is fully supported (:pull:`195`).
 - Python 3.8 support is dropped, as `it has reached end-of-life <https://peps.python.org/pep-0569/#lifespan>`__ (:pull:`195`).