confman #106247 plan for future attributes allocation (was: reserve a "pythonpath" attribute) [resolved]

not sure yet if this must be directly managed, but it should at least NOT go into the target repository .hgrc file

appeared in<not specified>
done in0.2.0
load left0.000
closed by#d46afa5e53c8 [attributes] be careful with the top-level namespace, create an `hgrc` key to handle the target hgrc entries (closes #106247)