future proofing is good, but...

since print() was made a function in py3, i can predict at some point
that return will be made a func as well. sure, good.
but "return()" *currently* returns an empty tuple. We want to
explicitly return None for testing purposes.

aif/disk/luks.py

@@ -40,7 +40,7 @@ class LuksSecretFile(LuksSecret):
             self.passphrase = secrets.token_bytes(self.size)
         if not isinstance(self.passphrase, bytes):
             self.passphrase = self.passphrase.encode('utf-8')
-        return()
+        return(None)
 
 
 class LUKS(object):
@@ -72,7 +72,7 @@ class LUKS(object):
                              '(aif.disk.luks.LuksSecretPassphrase or '
                              'aif.disk.luks.LuksSecretFile)')
         self.secrets.append(secretobj)
-        return()
+        return(None)
 
     def createSecret(self, secrets_xml = None):
         if not secrets_xml:  # Find all of them from self
@@ -116,11 +116,11 @@ class LUKS(object):
                                            passphrase = None,
                                            bytesize = kf.attrib.get('size', 4096))
             self.secrets.append(secretobj)
-        return()
+        return(None)
 
     def create(self):
         if self.created:
-            return()
+            return(None)
         if not self.secrets:
             raise RuntimeError('Cannot create a LUKS volume with no secrets added')
         for idx, secret in enumerate(self.secrets):
@@ -138,28 +138,28 @@ class LUKS(object):
                                                    self.secrets[0].passphrase,
                                                    secret.passphrase)
         self.created = True
-        return()
+        return(None)
 
     def lock(self):
         if not self.created:
             raise RuntimeError('Cannot lock a LUKS volume before it is created')
         if self.locked:
-            return()
+            return(None)
         _BlockDev.crypto.luks_close(self.name)
         self.locked = True
-        return()
+        return(None)
 
     def unlock(self, passphrase = None):
         if not self.created:
             raise RuntimeError('Cannot unlock a LUKS volume before it is created')
         if not self.locked:
-            return()
+            return(None)
         _BlockDev.crypto.luks_open_blob(self.source,
                                         self.name,
                                         self.secrets[0].passphrase,
                                         False)  # read-only
         self.locked = False
-        return()
+        return(None)
 
     def updateInfo(self):
         if self.locked:
@@ -177,7 +177,7 @@ class LUKS(object):
             info[k] = v
         info['_cipher'] = '{cipher}-{mode}'.format(**info)
         self.info = info
-        return()
+        return(None)
 
     def writeConf(self, conf = '/etc/crypttab'):
         if not self.secrets:
@@ -204,4 +204,4 @@ class LUKS(object):
         if luksinfo not in conflines:
             with open(conf, 'a') as fh:
                 fh.write('{0}\n'.format(luksinfo))
-        return()
+        return(None)