"Task exception was never retrieved" warning with Skipped exception

[blueyed]

The Skipped exception used by pytest (e.g. pytest.importorskip) is derived from BaseException, and therefore is not caught by asyncio.events's _run:

Ref: https://github.com/pytest-dev/pytest/blob/865e84d206581f77640cfbc1516aff7a0d7316c6/src/_pytest/outcomes.py#L36

https://github.com/blueyed/cpython/blob/443fe5a52a3d6a101795380227ced38b4b5e0a8b/Lib/asyncio/events.py#L79-L94

This results in the following warning/error when using it in an async test:

Task exception was never retrieved
future: <Task finished coro=<test_database_sync_to_async() done, defined at …/Vcs/channels/tests/test_db.py:6> exception=could not import 'psycopg2xx'>
Traceback (most recent call last):
  File "…/Vcs/channels/.venv/lib/python3.7/site-packages/_pytest/runner.py", line 226, in from_call
    result = func()
  File "…/Vcs/channels/.venv/lib/python3.7/site-packages/_pytest/runner.py", line 198, in <lambda>
    lambda: ihook(item=item, **kwds), when=when, reraise=reraise
  File "…/Vcs/channels/.venv/lib/python3.7/site-packages/pluggy/hooks.py", line 289, in __call__
    return self._hookexec(self, self.get_hookimpls(), kwargs)
  File "…/Vcs/channels/.venv/lib/python3.7/site-packages/pluggy/manager.py", line 68, in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
  File "…/Vcs/channels/.venv/lib/python3.7/site-packages/pluggy/manager.py", line 62, in <lambda>
    firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
  File "…/Vcs/channels/.venv/lib/python3.7/site-packages/pluggy/callers.py", line 208, in _multicall
    return outcome.get_result()
  File "…/Vcs/channels/.venv/lib/python3.7/site-packages/pluggy/callers.py", line 80, in get_result
    raise ex[1].with_traceback(ex[2])
  File "…/Vcs/channels/.venv/lib/python3.7/site-packages/pluggy/callers.py", line 187, in _multicall
    res = hook_impl.function(*args)
  File "…/Vcs/channels/.venv/lib/python3.7/site-packages/_pytest/runner.py", line 123, in pytest_runtest_call
    item.runtest()
  File "…/Vcs/channels/.venv/lib/python3.7/site-packages/_pytest/python.py", line 1464, in runtest
    self.ihook.pytest_pyfunc_call(pyfuncitem=self)
  File "…/Vcs/channels/.venv/lib/python3.7/site-packages/pluggy/hooks.py", line 289, in __call__
    return self._hookexec(self, self.get_hookimpls(), kwargs)
  File "…/Vcs/channels/.venv/lib/python3.7/site-packages/pluggy/manager.py", line 68, in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
  File "…/Vcs/channels/.venv/lib/python3.7/site-packages/pluggy/manager.py", line 62, in <lambda>
    firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
  File "…/Vcs/channels/.venv/lib/python3.7/site-packages/pluggy/callers.py", line 208, in _multicall
    return outcome.get_result()
  File "…/Vcs/channels/.venv/lib/python3.7/site-packages/pluggy/callers.py", line 80, in get_result
    raise ex[1].with_traceback(ex[2])
  File "…/Vcs/channels/.venv/lib/python3.7/site-packages/pluggy/callers.py", line 187, in _multicall
    res = hook_impl.function(*args)
  File "…/Vcs/pytest-asyncio/pytest_asyncio/plugin.py", line 158, in pytest_pyfunc_call
    pyfuncitem.obj(**testargs), loop=event_loop))
  File "/usr/lib/python3.7/asyncio/base_events.py", line 571, in run_until_complete
    self.run_forever()
  File "/usr/lib/python3.7/asyncio/base_events.py", line 539, in run_forever
    self._run_once()
  File "/usr/lib/python3.7/asyncio/base_events.py", line 1775, in _run_once
    handle._run()
  File "/usr/lib/python3.7/asyncio/events.py", line 88, in _run
    self._context.run(self._callback, *self._args)
  File "…/Vcs/channels/tests/test_db.py", line 15, in test_database_sync_to_async
    pytest.importorskip("psycopg2xx")
  File "…/Vcs/channels/.venv/lib/python3.7/site-packages/_pytest/outcomes.py", line 172, in importorskip
    raise Skipped(reason, allow_module_level=True)
Skipped: could not import 'psycopg2xx'

[jcrist]

Should be fixed by #126.