Bring in Qt import logic from `ipykernel`

See https://github.com/ipython/ipykernel/pull/1054

IPython/terminal/pt_inputhooks/__init__.py

@@ -40,6 +40,82 b' class UnknownBackend(KeyError):'
                                     ', '.join(backends + sorted(registered)))
 
 
+def set_qt_api(gui):
+    """Sets the `QT_API` environment variable if it isn't already set."""
+    # TODO: how do we do this here?
+    # if hasattr(kernel, "app"):
+    #     raise RuntimeError("Kernel already running a Qt event loop.")
+
+    # if gui != "qt" and hasattr(kernel, "last_qt_version"):
+    #     if kernel.last_qt_version != gui:
+    #         raise ValueError(
+    #             "Cannot switch Qt versions for this session; "
+    #             f"must use {kernel.last_qt_version}."
+    #         )
+
+    qt_api = os.environ.get("QT_API", None)
+    if qt_api is not None and gui != "qt":
+        env2gui = {
+            "pyside": "qt4",
+            "pyqt": "qt4",
+            "pyside2": "qt5",
+            "pyqt5": "qt5",
+            "pyside6": "qt6",
+            "pyqt6": "qt6",
+        }
+        if env2gui[qt_api] != gui:
+            print(
+                f'Request for "{gui}" will be ignored because `QT_API` '
+                f'environment variable is set to "{qt_api}"'
+            )
+    else:
+        if gui == "qt4":
+            try:
+                import PyQt  # noqa
+
+                os.environ["QT_API"] = "pyqt"
+            except ImportError:
+                try:
+                    import PySide  # noqa
+
+                    os.environ["QT_API"] = "pyside"
+                except ImportError:
+                    # Neither implementation installed; set it to something so IPython gives an error
+                    os.environ["QT_API"] = "pyqt"
+        elif gui == "qt5":
+            try:
+                import PyQt5  # noqa
+
+                os.environ["QT_API"] = "pyqt5"
+            except ImportError:
+                try:
+                    import PySide2  # noqa
+
+                    os.environ["QT_API"] = "pyside2"
+                except ImportError:
+                    os.environ["QT_API"] = "pyqt5"
+        elif gui == "qt6":
+            try:
+                import PyQt6  # noqa
+
+                os.environ["QT_API"] = "pyqt6"
+            except ImportError:
+                try:
+                    import PySide6  # noqa
+
+                    os.environ["QT_API"] = "pyside6"
+                except ImportError:
+                    os.environ["QT_API"] = "pyqt6"
+        elif gui == "qt":
+            # Don't set QT_API; let IPython logic choose the version.
+            if "QT_API" in os.environ.keys():
+                del os.environ["QT_API"]
+        else:
+            raise ValueError(
+                f'Unrecognized Qt version: {gui}. Should be "qt4", "qt5", "qt6", or "qt".'
+            )
+
+
 def get_inputhook_name_and_func(gui):
     print(f'`get_inputhook_name_and_func` called with {gui=}')
     if gui in registered:
@@ -49,20 +125,13 b' def get_inputhook_name_and_func(gui):'
         raise UnknownBackend(gui)
 
     if gui in aliases:
+        print('gui has an alias')
         return get_inputhook_name_and_func(aliases[gui])
 
     gui_mod = gui
-    if gui == "qt5":
-        os.environ["QT_API"] = "pyqt5"
-        gui_mod = "qt"
-    elif gui == "qt6":
-        # XXX: this locks us into pyqt6 even if pyside6 is installed.
-        os.environ["QT_API"] = "pyqt6"
+    if gui.startswith("qt"):
+        set_qt_api(gui)
         gui_mod = "qt"
 
-    print(f'{gui_mod=}')
-    # Note: `IPython.terminal.pt_inputhooks.qt` imports `IPython.external.qt_for_kernel` and that's
-    # where the environment variable locks us into `pyqt6`, despite the fact that it seems `PySide6`
-    # is supported.
-    mod = importlib.import_module('IPython.terminal.pt_inputhooks.'+gui_mod)
+    mod = importlib.import_module("IPython.terminal.pt_inputhooks." + gui_mod)
     return gui, mod.inputhook