bpo-33807: Add Python 3.7 buildbot link to '.github/CONTRIBUTING.rst' (GH-7532)

(cherry picked from commit 9e6685ea7b)

Co-authored-by: Suriyaa ✌️️ <isc.suriyaa@gmail.com>

.github/CONTRIBUTING.rst

@@ -8,6 +8,10 @@ Build Status
 
   + `Stable buildbots <http://buildbot.python.org/3.x.stable/>`_
 
+- 3.7
+
+  + `Stable buildbots <http://buildbot.python.org/3.7.stable/>`_
+
 - 3.6
 
   + `Stable buildbots <http://buildbot.python.org/3.6.stable/>`_