test_mail: simplify apt call

tests/test_email.py

@@ -14,7 +14,7 @@ import imp
 try:
     import spf
 except ImportError:
-    status, output = subprocess.getstatusoutput('apt-get -qq -y install python3-spf')
+    subprocess.call(['apt-get', '-qq', '-y', 'install', 'python3-spf'])
     import spf
 
 YELLOW = '\033[93m'