gh-35423: Fix doctests with ipython 8.12
    
### 📚 Description

As reported in
#35337 (comment), the
update to ipython 8.12 introduces some  useless output when calling
`get_ipython().enable_gui(...)` which causes a few doctest failures.

This PR silences this output by (temporarily) redirecting stdout to a
string.

This is only one commit that sits right on top of https://github.com/sag
emath/sage/commit/e9c3333d4145bf631f0eae91514219479e1000ef from #35337.

### 📝 Checklist

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [x] Existing tests already cover the changes.

### ⌛ Dependencies

- #35337
    
URL: #35423
Reported by: Gonzalo Tornaría
Reviewer(s): François Bissey

Author: Release Manager

src/sage/repl/inputhook.py

@@ -17,6 +17,8 @@
 
 import select
 import errno
+import contextlib
+import io
 
 from IPython import get_ipython
 from IPython.terminal.pt_inputhooks import register
@@ -65,7 +67,9 @@ def install():
     if not ip:
         return   # Not running in ipython, e.g. doctests
     if ip._inputhook != sage_inputhook:
-        ip.enable_gui('sage')
+        # silence `ip.enable_gui()` useless output
+        with contextlib.redirect_stdout(io.StringIO()):
+            ip.enable_gui('sage')
 
 
 def uninstall():
@@ -81,4 +85,6 @@ def uninstall():
     if not ip:
         return
     if ip._inputhook == sage_inputhook:
-        ip.enable_gui(None)
+        # silence `ip.enable_gui()` useless output
+        with contextlib.redirect_stdout(io.StringIO()):
+            ip.enable_gui(None)