Send only minor/major release announces to TIP mailing list

As discussed recently on the testing-in-python mailing list, people have asked 
to restrict the amount of announcements there only to major/minor 
pytest releases

HOWTORELEASE.rst

@@ -54,8 +54,8 @@ Note: this assumes you have already registered on pypi.
 11. Send release announcement to mailing lists:
 
     - pytest-dev@python.org
-    - testing-in-python@lists.idyll.org
     - python-announce-list@python.org
+    - testing-in-python@lists.idyll.org (only for minor/major releases)
 
     And announce the release on Twitter, making sure to add the hashtag ``#pytest``.