New patch (proxymixin.diff) uploaded that correctly delegates
__format__, as well as using an overridable return_inplace() method to
generate the inplace operation return values. The _target attribute has
also been made formally part of the public API (as 'target'), although
you obviously need to explicitly invoke object.__getattribute__ in order
for it to be visible. The name of the attribute is also available at the
module level as _PROXY_TARGET_ATTR.