------------------
**This is a breaking change.** Most implementation is replaced with
``contextvars``. In Python 3.5 and 3.6, ``aiocontextvars`` depends on
``contextvars`` the PEP-567 backport in PyPI, and patches it to partially
support asyncio; in Python 3.7 ``aiocontextvars`` is only a delegate to the
built-in ``contextvars`` library.
* Modified ``ContextVar.set()`` to return a token.
* Added ``ContextVar.reset(token)``.
* Removed ``ContextVar.delete()``.
* Removed ``enable_inherit()`` and ``disable_inherit()``, inherit is always enabled.
* Added ``copy_context()`` and ``Context.run()``.
* Removed ``Context.current()`` and ``Context.inherited``.
* Fixed issue that ``set_event_loop(None)`` fails (contributed by J.J. Jackson in 10 11)