Change hooks control (deny_all_hooks_but / allow_all_hooks_but) to be more predictable Prior to this, if one execute code like: with cnx.hooks.deny_all_hooks_but('metadata'): with cnx.hooks.deny_all_hooks_but(): # mycode 'metadata' hooks will be activated anyway in the inner block, which is rather unexpected (of course in real life you only see the latest hooks control statement, the former being higher in the call stack). This is due to the underlying usage of old `enable_hook_categories` / `disable_hook_categories` methods, which were introduced much before the now official context manager based API (with `cnx.[deny|all]_all_hooks_but(...)`). To move on, this patch drop the two legacy methods, rename and privatize related internal state on the connection (`hooks_mode` becomes `_hooks_mode`, `disabled_hook_cats` and `enabled_hook_cats` become `_hooks_categories`) and reimplement the `_hooks_control` context manager to simply update them. See the added unit test for details.

[cwconfig] make appobjects_cubes_modnames() public The method is used in cubicweb.cwvreg without underscore and outside of the config class, so let's make this method public. Fix autoreload with twisted.

Fix str(Unauthorized) which is really problematic on python 3 where __unicode__ is never called anymore. This was caused because in PY3, CubicWebException.__str__ was referencing __unicode__ implementation of CubicWebException, not of its subclass that implements it. Fix a flake8 style warning about lambda assignment as a bonus.