Various stylistic fixes

- Fix PEP8's line too long warning
- Remove useless parentheses
- Use triple quotes for docstring
- Address pylint's 'inconsistent-return-statements'
- Specify string format arguments as logging function parameters
- Comment out already disabled block of code
- Remove useless else (after return)
- Remove useless statement
- Do not import already imported module

whipper/common/cache.py

@@ -93,10 +93,10 @@ class Persister:
         self.object = default
 
         if not self._path:
-            return None
+            return
 
         if not os.path.exists(self._path):
-            return None
+            return
 
         handle = open(self._path)
         import pickle
@@ -109,7 +109,6 @@ class Persister:
             # can fail for various reasons; in that case, pretend we didn't
             # load it
             logger.debug(e)
-            pass
 
     def delete(self):
         self.object = None