Pkg.b2s: Pass through str as-is on Python 3

Things change, and we may start getting str where we previously got
bytes, such as what has apparently now happened with some versions of
the return value of magic.descriptor(fd) in Fedora Rawhide.

https://bugzilla.redhat.com/show_bug.cgi?id=1439941

Pkg.py

@@ -41,8 +41,8 @@
     unicode = str
 
     def b2s(b):
-        if b is None:
-            return None
+        if b is None or isinstance(b, str):
+            return b
         if isinstance(b, (list, tuple)):
             return [b2s(x) for x in b]
         return b.decode(errors='replace')

test/test.Pkg.py

@@ -30,6 +30,10 @@ def test_rangeCompare(self):
         ):
             self.assertFalse(Pkg.rangeCompare(req, prov))
 
+    def test_b2s(self):
+        for thing in ("foo", ["foo"]):
+            self.assertEqual(thing, Pkg.b2s(thing))
+
 
 if __name__ == '__main__':
     unittest.main()