component/postfix: version up 3.7.3

3.7.3 is an "urgent" new release as announced in
https://www.postfix.org/announcements/postfix-3.7.3.html
the old 3.7.2 version seem to have bee removed from mirrors
(see http://postfix.mirrors.ovh.net/postfix-release/official/ for a
mirror with directory index) and was not in shacache.

component/postfix/buildout.cfg

@@ -14,8 +14,8 @@ extends =
 [postfix]
 recipe = slapos.recipe.cmmi
 shared = true
-url = http://cdn.postfix.johnriley.me/mirrors/postfix-release/official/postfix-3.7.2.tar.gz
-md5sum = 92ff3863194004f7d8a0893421ba4248
+url = http://cdn.postfix.johnriley.me/mirrors/postfix-release/official/postfix-3.7.3.tar.gz
+md5sum = bbb2168f85530fe4eaa65408a48e731b
 location = @@LOCATION@@
 patch-options = -p1
 patches =