Suggestion: don't import uvicorn to allow for alternative asgi servers

The `core.py` file imports uvicorn even though it's only used for
`serve()` calls.

I'm deploying a FastHTML app to AWS Lambda, and extra imports cause
longer cold starts. I did a naive test (3 samples of each) and it seems
like this uvicorn import takes 10-25ms to load.

Moving the `uvicorn` import into `serve()` still works, and doesn't seem
to violate the project style guide.

Author: ryansb

fasthtml/core.py

@@ -11,7 +11,7 @@
            'MiddlewareBase', 'FtResponse', 'unqid', 'setup_ws']
 
 # %% ../nbs/api/00_core.ipynb
-import json,uuid,inspect,types,uvicorn,signal,asyncio,threading,inspect
+import json,uuid,inspect,types,signal,asyncio,threading,inspect
 
 from fastcore.utils import *
 from fastcore.xml import *
@@ -638,6 +638,7 @@ def serve(
     if not appname:
         if glb.get('__name__')=='__main__': appname = Path(glb.get('__file__', '')).stem
         elif code.co_name=='main' and bk.f_back.f_globals.get('__name__')=='__main__': appname = inspect.getmodule(bk).__name__
+    import uvicorn
     if appname:
         if not port: port=int(os.getenv("PORT", default=5001))
         print(f'Link: http://{"localhost" if host=="0.0.0.0" else host}:{port}')

nbs/api/00_core.ipynb

@@ -43,7 +43,7 @@
    "outputs": [],
    "source": [
     "#| export\n",
-    "import json,uuid,inspect,types,uvicorn,signal,asyncio,threading,inspect\n",
+    "import json,uuid,inspect,types,signal,asyncio,threading,inspect\n",
     "\n",
     "from fastcore.utils import *\n",
     "from fastcore.xml import *\n",
@@ -1582,6 +1582,7 @@
     "    if not appname:\n",
     "        if glb.get('__name__')=='__main__': appname = Path(glb.get('__file__', '')).stem\n",
     "        elif code.co_name=='main' and bk.f_back.f_globals.get('__name__')=='__main__': appname = inspect.getmodule(bk).__name__\n",
+    "    import uvicorn\n",
     "    if appname:\n",
     "        if not port: port=int(os.getenv(\"PORT\", default=5001))\n",
     "        print(f'Link: http://{\"localhost\" if host==\"0.0.0.0\" else host}:{port}')\n",