add missing changelog entry: Make ``python-dateutil`` a soft-dependency.

CHANGES.rst

@@ -5,6 +5,9 @@ Changelog
 3.6.3 (unreleased)
 ------------------
 
+- Make ``python-dateutil`` a soft-dependency.
+  [boltnev]
+
 - Add optional ``sorted`` parameter to ``Component.to_ical``. Setting it to
   false allows the user to preserve the original property and parameter order.
   [untitaker]

setup.py

@@ -10,10 +10,11 @@ longdesc += codecs.open('CHANGES.rst', encoding='utf-8').read()
 longdesc += codecs.open('LICENSE.rst', encoding='utf-8').read()
 
 
-tests_require = []
+tests_require = [
+    'python-dateutil',
+]
 install_requires = [
     'setuptools',
-    'python-dateutil',
     'pytz'
 ]