Update contextlib2 to 21.6.0

[pyup-bot]

This PR updates contextlib2 from 0.6.0.post1 to 21.6.0.

Changelog

21.6.0

^^^^^^^^^^^^^^^^^^^

* License update: due to the inclusion of type hints from the ``typeshed``
project, the ``contextlib2`` project is now under a combination of the
Python Software License (existing license) and the Apache License 2.0
(``typeshed`` license)
* Switched to calendar based versioning using a "year"-"month"-"serial" scheme,
rather than continuing with pre-1.0 semantic versioning
* Due to the inclusion of asynchronous features from Python 3.7+, the
minimum supported Python version is now Python 3.6
(`29 <https://github.com/jazzband/contextlib2/issues/29>`__)
* Synchronised with the Python 3.10 version of contextlib
(`12 <https://github.com/jazzband/contextlib2/issues/12>`__), making the
following new features available on Python 3.6+:

* ``asyncontextmanager`` (added in Python 3.7, enhanced in Python 3.10)
* ``aclosing`` (added in Python 3.10)
* ``AbstractAsyncContextManager`` (added in Python 3.7)
* ``AsyncContextDecorator`` (added in Python 3.10)
* ``AsyncExitStack`` (added in Python 3.7)
* async support in ``nullcontext`` (Python 3.10)

* ``contextlib2`` now includes an adapted copy of the ``contextlib``
type hints from ``typeshed`` (the adaptation removes the Python version
dependencies from the API definition)
(`33 <https://github.com/jazzband/contextlib2/issues/33>`__)
* to incorporate the type hints stub file and the ``py.typed`` marker file,
``contextlib2`` is now installed as a package rather than as a module
* Updates to the default compatibility testing matrix:

* Added: CPython 3.9, CPython 3.10
* Dropped: CPython 2.7, CPython 3.5, PyPy2

Links

• PyPI: https://pypi.org/project/contextlib2
• Changelog: https://pyup.io/changelogs/contextlib2/
• Docs: http://contextlib2.readthedocs.org