Run_sync (v3) raises `AsyncLibraryNotFoundError` for python3.6

Problem

The AsyncLibraryNotFoundError is getting raised for running a synchronous function in a worker thread in py3.6.

For more context, Jupyter Server #505

How to reproduce

Python version: Python 3.6.13 :: Anaconda, Inc.

Confirm AsyncLibraryNotFoundError isn’t raised for anyio==2.2.0.

$ pip install anyio==2.2.0
$ python test_run_sync.py

AsyncLibraryNotFoundError shows up for anyio==3.0.1 in python3.6. I ran this in python3.7, and the error didn’t show up.

$ pip install anyio==3.0.1
$ python test_run_sync.py

Exception in callback WorkerThread.stop(<Task finishe...> result=None>)
handle: <Handle WorkerThread.stop(<Task finishe...> result=None>)>
Traceback (most recent call last):
  File "/Users/mwaka/opt/anaconda3/envs/jp_test_3.6/lib/python3.6/asyncio/events.py", line 145, in _run
    self._callback(*self._args)
  File "/Users/mwaka/opt/anaconda3/envs/jp_test_3.6/lib/python3.6/site-packages/anyio/_backends/_asyncio.py", line 708, in stop
    _threadpool_workers.get().discard(self)
  File "/Users/mwaka/opt/anaconda3/envs/jp_test_3.6/lib/python3.6/site-packages/anyio/lowlevel.py", line 113, in get
    return self._current_vars[self._name]
  File "/Users/mwaka/opt/anaconda3/envs/jp_test_3.6/lib/python3.6/site-packages/anyio/lowlevel.py", line 97, in _current_vars
    token = current_token()
  File "/Users/mwaka/opt/anaconda3/envs/jp_test_3.6/lib/python3.6/site-packages/anyio/lowlevel.py", line 54, in current_token
    return get_asynclib().current_token()
  File "/Users/mwaka/opt/anaconda3/envs/jp_test_3.6/lib/python3.6/site-packages/anyio/_core/_eventloop.py", line 109, in get_asynclib
    asynclib_name = sniffio.current_async_library()
  File "/Users/mwaka/opt/anaconda3/envs/jp_test_3.6/lib/python3.6/site-packages/sniffio/_impl.py", line 82, in current_async_library
    "unknown async library, or not in async context"
sniffio._impl.AsyncLibraryNotFoundError: unknown async library, or not in async context

The test_run_sync.py script: https://gist.github.com/mwakaba2/e76b0bc341bf17c97ff58cd4c672e4d4

[agronholm]

Yes, the test fails without the fix.

[mwakaba2]

@agronholm Thanks so much for the quick fix!

In your PR, I see a test that checks for any exceptions in run_sync. Were you able to reproduce the exception before the fix?