Cannot run `shiny run` with `--reload`

[gadenbuie]

After #1183, running shiny run --reload results in an error when trying to cast(ASGI3Application, app)

py-shiny/shiny/_autoreload.py

Line 107 in 0b170d3

self.app = cast(ASGI3Application, app)

INFO:     Uvicorn running on http://127.0.0.1:52287 (Press CTRL+C to quit)
INFO:     Started reloader process [75586] using WatchFiles
INFO:     Started server process [75591]
INFO:     Waiting for application startup.
ERROR:    Exception in 'lifespan' protocol
Traceback (most recent call last):
  File "/Users/garrick/work/posit-dev/py-shiny/venv/lib/python3.11/site-packages/uvicorn/lifespan/on.py", line 86, in main
    await app(scope, self.receive, self.send)
  File "/Users/garrick/work/posit-dev/py-shiny/venv/lib/python3.11/site-packages/uvicorn/middleware/proxy_headers.py", line 84, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/garrick/work/posit-dev/py-shiny/shiny/_app.py", line 275, in __call__
    await self.starlette_app(scope, receive, send)
  File "/Users/garrick/work/posit-dev/py-shiny/venv/lib/python3.11/site-packages/starlette/applications.py", line 122, in __call__
    self.middleware_stack = self.build_middleware_stack()
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/garrick/work/posit-dev/py-shiny/venv/lib/python3.11/site-packages/starlette/applications.py", line 109, in build_middleware_stack
    app = cls(app=app, *args, **kwargs)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/garrick/work/posit-dev/py-shiny/shiny/_autoreload.py", line 107, in __init__
    self.app = cast(ASGI3Application, app)
                    ^^^^^^^^^^^^^^^^
NameError: name 'ASGI3Application' is not defined
ERROR:    Application startup failed. Exiting.

cc @wch

[wch]

This was due to changes I made in #1183: it loaded some types only when typing.TYPE_CHECKING is true, so that those types were only loaded during the static analysis phase, but later in the code it used cast() with some of those types, and that happens at run time, when types aren't loaded. I have a fix where the casting only occurs when TYPE_CHECKING is true.

I wish Python's type system was more consistent in how it worked, and was completely ignored at run time, instead of being mostly being ignored but with a few exceptions like cast().