This requires supporting sphinx-issues to match the Pallets practice
of adding the pr/issue number in the changelog.

This allows for signals to send to coroutine receivers by awaiting
them. The _async_wrapper and _sync_wrapper allows for conversion of
sync and async receivers as required if defined. If not defined a
runtime error is raised.

The wrappers are used to avoid any direct tie into asyncio, trio,
greenbacks, asgiref, or other specific async implementation.

This is useful whilst testing to remove a signal's affects.

Co-authored-by: Michael Elovskikh <wronglink@yandex-team.ru>