style: lint with 'black'

Author: srevinsaju

aslo4/lib/progressbar/widgets.py

@@ -843,7 +843,7 @@ def __call__(self, progress, data):
 
 
 class VariableMixin(object):
-    """Mixin to display a custom user variable """
+    """Mixin to display a custom user variable"""
 
     def __init__(self, name, **kwargs):
         if not isinstance(name, six.string_types):