Configtree

---

* Dropped deprecated features.
* Fixed deprecation warnings on Python 3.7 and higher.
* Migrated tests from ``Nose`` to ``PyTest``.

-----

* Fixed bug in ``Walker.environment`` method.

-----

* Fixed bugs in ``ITree.rare_copy`` and ``ITree.rare_keys`` methods.

-----

* Fixed bugs in ``Loader`` class.

---

* Added abstract base class ``ITree`` to unify type checking;
* Fixed ``pop`` method of ``Tree`` and ``BranchProxy``;
* Added ``rare_copy`` method into ``Tree`` and ``BranchProxy``;
* Unified ``rarefy`` function, it now handles any mapping object.

---

* Dropped Python 2.6 support.
* Completely reworked loading process (see `migration guide`_):

* functions ``load``, ``loaderconf`` are deprecated in favor of class ``Loader``;
* function ``make_walk`` is deprecated in favor of ``Walker``;
* function ``make_update`` is deprecated in favor of ``Updater``;
* module ``configtree.conv`` and its plugins (from entry point with
the same name) is deprecated in favor or ``configtree.formatter``;
* shell command ``configtree`` is deprecated in favor of ``ctdump``.


.. _migration guide: http://configtree.readthedocs.org/en/latest/migration.html
migration-from-version-0-3-to-0-4