handle bytes in a py3 compatible way in confit

This applies to Subview and Filename.

However, the Filename support isn't complete, since we can't
implicitly convert bytes to strings in py3. Confit doesn't
seem to have a way to handle this.

beets/util/confit.py

@@ -481,11 +481,11 @@ def __init__(self, parent, key):
                 self.name += '.'
         if isinstance(self.key, int):
             self.name += u'#{0}'.format(self.key)
-        elif isinstance(self.key, BASESTRING):
-            if isinstance(self.key, bytes):
-                self.name += self.key.decode('utf8')
-            else:
+        elif isinstance(self.key, (bytes, BASESTRING)):
+            if isinstance(self.key, STRING):
                 self.name += self.key
+            else:
+                self.name += self.key.decode('utf8')
         else:
             self.name += repr(self.key)
 
@@ -1341,7 +1341,7 @@ def resolve_relative_to(self, view, template):
 
     def value(self, view, template=None):
         path, source = view.first()
-        if not isinstance(path, BASESTRING):
+        if not isinstance(path, (bytes, BASESTRING)):
             self.fail(
                 u'must be a filename, not {0}'.format(type(path).__name__),
                 view,