python: Add Django 5.x compatibility

[ac000]

    python: Add Django 5.x compatibility
    
    Note: This may not be *specific* to Django 5.x but is where the issue
    showed up.
    
    @codedoga on GitHub reported an issue with Unit and Django 5.x
    
    When trying to perform a simple POST/PUT request with body data, Unit
    was throwing the following error
    
      2025/02/16 11:07:14 [error] 6#6 [unit] #9: Python failed to call 'future.result()'
      Traceback (most recent call last):
        File "/usr/local/lib/python3.13/site-packages/django/core/handlers/asgi.py", line 162, in __call__
          await self.handle(scope, receive, send)
        File "/usr/local/lib/python3.13/site-packages/django/core/handlers/asgi.py", line 208, in handle
          task.result()
          ~~~~~~~~~~~^^
        File "/usr/local/lib/python3.13/site-packages/django/core/handlers/asgi.py", line 239, in listen_for_disconnect
          assert False, "Invalid ASGI message after request body: %s" % message["type"]
                 ^^^^^
      AssertionError: Invalid ASGI message after request body: http.request
    
    There is no such issue with Django 4.x
    
    The issue was caused when Django started doing an 'async receive()' just
    after we have handled the initial request and passed it to the
    application. Django is then looking to see if/when we send it a
    'http.disconnect' message.
    
    We were not prepared for this and would go through all the motions of
    handling the request again which would result in the erroneous
    'http.request' message.
    
    What we need to do is track when we've handled the initial request. We
    can then use that information coupled with the fact if we get a request
    with 0 content length then we basically have nothing to do.
    
    For this we create a new nxt_py_asgi_http_t member, request_received.
    
    We can repurpose 'empty_body_received' for this if we rename it and
    change where we set it as now if 'request_received' is true then so
    would 'empty_body_received'.
    
    'empty_body_received' was actually part of a previous commit that was
    addressing various receive() issues. I've checked that the provided
    reproducer application still works.
    
    Link: <https://github.com/django/django/commit/1d1ddffc27cd55c011298cd09bfa4de3fa73cf7a>
    Link: <https://github.com/nginx/unit/issues/564>
    Fixes: 567545213 ("Python: fixing ASGI receive() issues.")
    Closes: https://github.com/nginx/unit/issues/1561
    Signed-off-by: Andrew Clayton <[email protected]>

[hongzhidao]

Looks good.