Support async file types and two context managers

[Mayst0731]

Hilloooo，

#1620.
I tried to deal with this issue and wanna try to run the code below first to see what's really going on.

async def main():
    async with httpx.AsyncClient() as client:
        async with trio.Path('./content.txt') as f:
            client.post("https://www.google.com/", content=f)
   
trio.run(main)

The error shows like

Traceback (most recent call last):
  File "file_doc.py", line 23, in <module>
    trio.run(main)
  File "/usr/local/lib/python3.8/site-packages/trio/_core/_run.py", line 1932, in run
    raise runner.main_task_outcome.error
  File "file_doc.py", line 19, in main
    async with trio.Path('./content.txt') as f:
AttributeError: __aexit__

1. I think the first step would be fixing this issue first, am I going far from this issue currently?

[graingert]

You need async with trio.Path('./content.txt').open("rb") as f: