Fixed AssertionError when using nest_asyncio

[agronholm]

Changes

Refactored TaskStateStore to not check explicitly against asyncio.Task, as nest-asyncio replaces the asyncio modules.

Fixes #840.

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) added which would fail without your patch
• You've updated the documentation (in docs/, in case of behavior changes or new
  features)
• You've added a new changelog entry (in docs/versionhistory.rst).

If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
these instructions.

Updating the changelog

If there are no entries after the last release, use **UNRELEASED** as the version.
If, say, your patch fixes issue #123, the entry should look like this:

- Fix big bad boo-boo in task groups
  (`#123 <https://github.com/agronholm/anyio/issues/123>`_; PR by @yourgithubaccount)

If there's no issue linked, just link to your pull request instead by updating the
changelog after you've created the PR.

[agronholm]

I don't know how to write a test for this, as nest_asyncio.apply() is too invasive to be used in the test suite.

[graingert]

you could try with:

with mock.patch("asyncio.Task", asyncio.tasks._PyTask):
    ...

[agronholm]

I've added a test and verified that it fails without this patch.

[edgarrmondragon]

I can confirm that this patch fixes the problem for us

[sigridjineth]

@agronholm I just have came across this, but would be better implementing the logic for accomodating some varities of task-like objects I think. what's your opinion?

[agronholm]

@agronholm I just have came across this, but would be better implementing the logic for accomodating some varities of task-like objects I think. what's your opinion?

If you find a problem not fixed by this PR, let me know. Otherwise I'm not inclined to add any more code just because.

[sandangel]

Hi, thank you so much for providing the fix for this issue. I wonder if we can create a release for this so I can download?

[agronholm]

I'm hoping to tackle #805 and #847 and then cut a new release. If those take too long, however, I'll just defer them.

[sandangel]

Please take your time. I pin to 4.6.2 at the moment.