text: Title: [Trio] Task groups are marked inactive early
  Body: Code:
  ```py
  import anyio
  
  
  async def second():
      print("This never happens")
  
  
  async def first(tg):
      await anyio.sleep(0)
      await tg.spawn(second)
  
  
  async def main():
      async with anyio.create_task_group() as tg:
          await tg.spawn(first, tg)
  
  
  anyio.run(main, backend="trio")
  ```
  
  Error:
  ```py
  Traceback (most recent call last):
    File "/home/laura/dev/discord/curious/aaa.py", line 18, in <module>
      anyio.run(main, backend="trio")
    File "/home/laura/.local/share/virtualenvs/curious-yiBjxk2Q/lib/python3.7/site-packages/anyio/__init__.py", line 112, in run
      return asynclib.run(func, *args, **backend_options)
    File "/home/laura/.local/share/virtualenvs/curious-yiBjxk2Q/lib/python3.7/site-packages/trio/_core/_run.py", line 1337, in run
      raise runner.main_task_outcome.error
    File "/home/laura/dev/discord/curious/aaa.py", line 15, in main
      await tg.spawn(first, tg)
    File "/home/laura/.local/share/virtualenvs/curious-yiBjxk2Q/lib/python3.7/site-packages/async_generator/_util.py", line 42, in __aexit__
      await self._agen.asend(None)
    File "/home/laura/.local/share/virtualenvs/curious-yiBjxk2Q/lib/python3.7/site-packages/async_generator/_impl.py", line 366, in step
      return await ANextIter(self._it, start_fn, *args)
    File "/home/laura/.local/share/virtualenvs/curious-yiBjxk2Q/lib/python3.7/site-packages/async_generator/_impl.py", line 202, in send
      return self._invoke(self._it.send, value)
    File "/home/laura/.local/share/virtualenvs/curious-yiBjxk2Q/lib/python3.7/site-packages/async_generator/_impl.py", line 209, in _invoke
      result = fn(*args)
    File "/home/laura/.local/share/virtualenvs/curious-yiBjxk2Q/lib/python3.7/site-packages/anyio/_backends/trio.py", line 90, in create_task_group
      tg._active = False
    File "/home/laura/.local/share/virtualenvs/curious-yiBjxk2Q/lib/python3.7/site-packages/trio/_core/_run.py", line 397, in __aexit__
      raise combined_error_from_nursery
    File "/home/laura/dev/discord/curious/aaa.py", line 10, in first
      await tg.spawn(second)
    File "/home/laura/.local/share/virtualenvs/curious-yiBjxk2Q/lib/python3.7/site-packages/anyio/_backends/trio.py", line 78, in spawn
      raise RuntimeError('This task group is not active; no new tasks can be spawned.')
  RuntimeError: This task group is not active; no new tasks can be spawned.
  ```
  
  Both curio and asyncio backends allow this to work.