Python 3.2 workaround: Replace FileNotFoundError with {IO,OS}Error

The FileNotFound class is not yet available in Python 3.2, but as long as
snakefruit still runs Ubuntu 12.04 we need to get along without it.

britney.py

@@ -480,7 +480,7 @@ class Britney(object):
         try:
             release_file = read_release_file(self.suite_info['testing'].path)
             self.log("Found a Release file in testing - using that for defaults")
-        except FileNotFoundError:
+        except IOError:
             self.log("Testing does not have a Release file.")
             release_file = None
 

britney2/policies/policy.py

@@ -320,7 +320,7 @@ class AgePolicy(BasePolicy):
                         dates[l[0]] = (l[1], int(l[2]))
                     except ValueError:
                         pass
-        except FileNotFoundError:
+        except IOError:
             if not using_new_name:
                 # If we using the legacy name, then just give up
                 raise

britney2/utils.py

@@ -660,7 +660,7 @@ def possibly_compressed(path, permitted_compressesion=None):
         cpath = "%s.%s" % (path, ext)
         if os.path.exists(cpath):
             return cpath
-    raise FileNotFoundError(errno.ENOENT, os.strerror(errno.ENOENT), path)
+    raise OSError(errno.ENOENT, os.strerror(errno.ENOENT), path)
 
 
 def create_provides_map(packages):