Use LOG.warning instead of deprecated LOG.warn

The LOG.warn method is deprecated[1] and the LOG.warning method should
be used instead.

[1] https://docs.python.org/3/library/logging.html#logging.warning

Change-Id: I43e2269d4b1c425119c86919eb869941d621c6de
diff --git a/HACKING.rst b/HACKING.rst
index 95bcbb5..dc28e4e 100644
--- a/HACKING.rst
+++ b/HACKING.rst
@@ -26,6 +26,7 @@
 - [T116] Unsupported 'message' Exception attribute in PY3
 - [T117] Check negative tests have ``@decorators.attr(type=['negative'])``
   applied.
+- [T118] LOG.warn is deprecated. Enforce use of LOG.warning.
 
 It is recommended to use ``tox -eautopep8`` before submitting a patch.