AssertionError when self.connection is not None in handle_async_request / _attempt_to_acquire_connection

[Zac-HD]

Traceback (most recent call last):
  ...
  File "/python3.10/site-packages/httpx/_client.py", line 1848, in post
    return await self.request(
  File "/python3.10/site-packages/httpx/_client.py", line 1533, in request
    return await self.send(request, auth=auth, follow_redirects=follow_redirects)
  File "/python3.10/site-packages/httpx/_client.py", line 1620, in send
    response = await self._send_handling_auth(
  File "/python3.10/site-packages/httpx/_client.py", line 1648, in _send_handling_auth
    response = await self._send_handling_redirects(
  File "/python3.10/site-packages/httpx/_client.py", line 1685, in _send_handling_redirects
    response = await self._send_single_request(request)
  File "/python3.10/site-packages/httpx/_client.py", line 1722, in _send_single_request
    response = await transport.handle_async_request(request)
  File "/python3.10/site-packages/httpx/_transports/default.py", line 353, in handle_async_request
    resp = await self._pool.handle_async_request(req)
  File "/python3.10/site-packages/httpcore/_async/connection_pool.py", line 221, in handle_async_request
    await self._attempt_to_acquire_connection(status)
  File "/python3.10/site-packages/httpcore/_async/connection_pool.py", line 160, in _attempt_to_acquire_connection
    status.set_connection(connection)
  File "/python3.10/site-packages/httpcore/_async/connection_pool.py", line 22, in set_connection
    assert self.connection is None
AssertionError

Like #615 this was fine in 0.15 but broken in 0.16.1; it might also be related to #613. I don't have a good repro yet.

[tomchristie]

Closed by #627 and released as 0.16.2