loop: use python 3 style base class initialization

Use the canonical Python3 usage of "super" without any arguments.
pylint 2.6.0 started to actually warn about this.

osbuild/loop.py

@@ -14,7 +14,7 @@ __all__ = [
 
 class UnexpectedDevice(Exception):
     def __init__(self, expected_minor, rdev, mode):
-        super(UnexpectedDevice, self).__init__()
+        super().__init__()
         self.expected_minor = expected_minor
         self.rdev = rdev
         self.mode = mode