aentering a MagicMock() results in an AsyncMock which behaves differently than I expected:

```
python3.9 -m asyncio
asyncio REPL 3.9.0a5 (default, Apr 18 2020, 00:00:31) 
[GCC 9.3.0] on linux
Use "await" directly instead of "asyncio.run()".
Type "help", "copyright", "credits" or "license" for more information.
>>> import asyncio
>>> from unittest import mock
>>> with mock.MagicMock() as m:
...     with m.foo() as u:
...         u.hello()
... 
<MagicMock name='mock.__enter__().foo().__enter__().hello()' id='140670894620048'>
>>> async with mock.MagicMock() as m:
...     async with m.foo() as u:
...         u.hello()
... 
<console>:2: RuntimeWarning: coroutine 'AsyncMockMixin._execute_mock_call' was never awaited
RuntimeWarning: Enable tracemalloc to get the object allocation traceback
Traceback (most recent call last):
  File "/usr/lib/python3.9/concurrent/futures/_base.py", line 439, in result
    return self.__get_result()
  File "/usr/lib/python3.9/concurrent/futures/_base.py", line 388, in __get_result
    raise self._exception
  File "<console>", line 2, in <module>
AttributeError: __aenter__
```

This is annoying for mocking database interfaces like

```
async def update_users(db, user):
    async with db.connection() as conn:
        async with conn.transaction() as tx:
            ...
```