Marks (temporarily) the next release as 1.2.4
Puts context's data in a stacked-dicts structure.

Now inner context's data does not overwrite upper levels, but inner level can
inspect (but not alter) upper levels.
Regains pypy testability: annotation by signature should not be tested on PyPy.
Documents the newly stacked context's data.
Makes the :attr:`xoutil.context.Context.data` an "open stacked dict"
Completes the implementation of the opendict for context's data: Now you can assign the data.
Fixes stackeddict so that it has a get method
Adds a always empty `data` attribute for NullContext
New flextime class, only to allow quick shell-based calculations with times
RestText format.
Add a FIXME for an unfinished feature.
Separating data for context levels.
Merge de docs con manu.
Modificaciones parciales de los contextos.
Finishes the first version of xoutil.collections.StackedDict
Fixes :func:`xoutil.decorator.compat.metaclass` so that slots work
Fixes OpenDictMixin to take account of slots in all the hierarchy
Fixes StackedDict API
Updates contexts to be stacked dicts. Removes the `data` attribute from
contexts.
Fixes opendict implementation to avoid methods shadowing.

The previous implementation allowed to hide dict's method like get,
__getitem__, etc..

Since __getattr__ is only called if such an attribute does not exists, it makes
sence a simple implementation like the one this commit introduces.

Alternatively one may set __dict__ to self, but that requires a sure round to
__getattribute__ to avoid the methods shadowing. So, the suppossed efficiency
gained in direct access to __self__ is lost by calling __getattribute__.