- update to 4.1.0:
* ``Projection`` now accepts an iterable or callable or pattern
for matching keys. ``Projection`` now retains order of keys
from the underlying mapping.
* ``DictFilter`` is now deprecated in favor of ``Projection``.
* ``DictFilter`` no longer accepts ``include_keys`` and
requires ``include_pattern`` as a keyword argument.
* In ``DictFilter``, deprecated ``include_keys`` parameter and
usage without ``include_pattern``. Future versions will honor
``include_pattern`` as a required keyword argument. All other
uses are deprecated. For uses that currently rely on
``include_keys``, use ``Projection`` instead/in addition.
* ``DictFilter.__len__`` no longer relies on the iterable.
Improves efficiency and fixes ``RecursionError`` on PyPy (#12).