gh-35337: Fix warning with ipython 8.11 (was #35235)

    
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### 📚 Description

ipython 8.11 throws new warnings "UserWarning: Shell was already running
a gui event loop for sage; switching to sage." when running some tests.

This is because sage indeed tries to install the same gui event loop for
ipython again and again. The fix is checking if the ipython input hook
is already set to the sage input hook, in that case we don't install the
gui event loop again.

In the test, we run `install()` twice so we can verify that running it
multiple times won't cause a warning anymore.

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### Replaces: #35235
Since gh doesn't allow to change the branch on a PR.
    
URL: #35337
Reported by: Gonzalo Tornaría
Reviewer(s): François Bissey

src/sage/repl/inputhook.py

@@ -47,15 +47,25 @@ def install():
     """
     Install the Sage input hook
 
-    EXAMPLES::
+    EXAMPLES:
+
+    Make sure ipython is running so we really test this function::
+
+        sage: from sage.repl.interpreter import get_test_shell
+        sage: get_test_shell()
+        <sage.repl.interpreter.SageTestShell object at ...>
+
+    Run the function twice, to check it is idempotent (see :trac:`35235`)::
 
         sage: from sage.repl.inputhook import install
         sage: install()
+        sage: install()
     """
     ip = get_ipython()
     if not ip:
         return   # Not running in ipython, e.g. doctests
-    ip.enable_gui('sage')
+    if ip._inputhook != sage_inputhook:
+        ip.enable_gui('sage')
 
 
 def uninstall():