fix: no hard dependency on nest_asyncio, use only when needed.

Only when we have a current event loop do we need nest_asyncio to be
used and installed. In case where readinto is called from a thread
(this is something we do in solara) which does not have an event loop
yet, we should not need nest_asyncio.

When needed, and not installed, we raise an error with an informative
installation instruction.

ipyvuetify/extra/file_input.py

@@ -5,7 +5,14 @@
 import sys
 
 import IPython
-import nest_asyncio
+
+try:
+    import nest_asyncio
+
+    has_nest_asyncio = True
+except ModuleNotFoundError:
+    has_nest_asyncio = False
+
 import traitlets
 
 import ipyvuetify as v
@@ -173,7 +180,12 @@ def __init__(self, **kwargs):
             return
         kernel = IPython.get_ipython().kernel
         if kernel.implementation == "ipython":
-            nest_asyncio.apply()
+            if not has_nest_asyncio:
+                raise RuntimeError(
+                    "nest_asyncio is required for FileInput in IPython kernel, please run 'pip install nest_asyncio'"
+                )
+            else:
+                nest_asyncio.apply()
 
     @traitlets.observe("file_info")
     def _file_info_changed(self, _):