rss2email module: init

Also adding `system-sendmail` package for sharing the code with other
modules or packages needing it.

pkgs/applications/networking/feedreaders/rss2email/default.nix

@@ -1,11 +1,13 @@
-{ pythonPackages, fetchurl, lib }:
+{ pythonPackages, fetchurl, lib, nixosTests }:
 
 with pythonPackages;
 
 buildPythonApplication rec {
   name = "${pname}-${version}";
   pname = "rss2email";
-  version = "3.9";
+  version = "3.9"; # TODO: on next bump, the manpage will be updated.
+  # Update nixos/modules/services/mail/rss2email.nix to point to it instead of
+  # to the online r2e.1
 
   propagatedBuildInputs = [ feedparser beautifulsoup4 html2text ];
 
@@ -44,4 +46,7 @@ buildPythonApplication rec {
     license = licenses.gpl2;
     maintainers = with maintainers; [ jb55 Profpatsch ];
   };
+  passthru.tests = {
+    smoke-test = nixosTests.rss2email;
+  };
 }