jupyter_server: Anyio version v3 breaks AsyncContentsManager for python3.6

Description

AsyncFileContentsManager tests are timing out in a python 3.6 environment due to a AsyncLibraryNotFoundError error.

DEBUG    ServerApp:log.py:54 200 GET /a%40b/api/contents/foo/a.txt?content=1 (127.0.0.1) 2.04ms
ERROR    asyncio:base_events.py:1296 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

Reproduce

$ pytest -vv jupyter_server/tests/services/contents/test_api.py

Expected behavior

The tests should pass in ~10 seconds.

pytest -vv jupyter_server/tests/services/contents/test_api.py  6.81s user 2.50s system 89% cpu 10.358 total

Context

• Operating System and version: MacOS, 10.15.7 (Catalina)
• Python version: Python 3.6.13 |Anaconda, Inc.| (default, Feb 23 2021, 12:58:59)