Fix docs build: resolve InteractiveShell forward reference in KernelMagics

sphinx_autodoc_typehints cannot resolve inherited type annotations from
parent classes in a different module. KernelMagics inherits shell: InteractiveShell
from IPython's Magics, but sphinx evaluates it in the parent module's namespace.

Re-declare the annotation in KernelMagics so sphinx resolves it in zmqshell.py
where InteractiveShell is imported.

Author: erikgaas

ipykernel/zmqshell.py

@@ -251,6 +251,7 @@ def unregister_hook(self, hook):
 @magics_class
 class KernelMagics(Magics):
     """Kernel magics."""
+    shell: InteractiveShell | None  # already imported in zmqshell.py
 
     # ------------------------------------------------------------------------
     # Magic overrides