config: reject str sections and keys on Python 3

Otherwise we could end up with a dict having both the str
and bytes versions of a particular config item. This may cause
some tests to regress. I haven't checked. But it is better
behavior to fail fast.

We could just as easily change this to normalize the values. But
I like catching all non-compliant call sites first.

Differential Revision: https://phab.mercurial-scm.org/D5649

mercurial/config.py

@@ -78,6 +78,10 b' class config(object):'
         return list(self._data.get(section, {}).iteritems())
     def set(self, section, item, value, source=""):
         if pycompat.ispy3:
+            assert not isinstance(section, str), (
+                'config section may not be unicode strings on Python 3')
+            assert not isinstance(item, str), (
+                'config item may not be unicode strings on Python 3')
             assert not isinstance(value, str), (
                 'config values may not be unicode strings on Python 3')
         if section not in self: