Define the IPython magic lazily

IPython is quite a chunky package, and importing it unconditionally makes
jaxtyping itself relatively slow to import.

jaxtyping/_ipython_extension.py

@@ -20,7 +20,9 @@
 from ._import_hook import JaxtypingTransformer, Typechecker
 
 
-try:
+def choose_typechecker_magics():
+    # The import is local to avoid degrading import times when the magic is
+    # not needed.
     from IPython.core.magic import line_magic, Magics, magics_class
 
     @magics_class
@@ -40,17 +42,15 @@ def typechecker(self, typechecker):
                 JaxtypingTransformer(typechecker=Typechecker(typechecker))
             )
 
-except Exception:
-    # Very broad exception-handling, as e.g. IPython will sometimes be
-    # present but fail to import for mysterious reasons.
-    pass
+    return ChooseTypecheckerMagics
 
 
 def load_ipython_extension(ipython):
     try:
-        ipython.register_magics(ChooseTypecheckerMagics)
-    except NameError:
-        raise NameError(
-            "ChooseTypecheckerMagics is not defined.\n\n"
-            + "You may be trying to use IPython extension without IPython installed."
-        )
+        ChooseTypecheckerMagics = choose_typechecker_magics()
+    except Exception as e:
+        # Very broad exception-handling, as e.g. IPython will sometimes be
+        # present but fail to import for mysterious reasons.
+        raise RuntimeError("Failed to define jaxtyping.typechecker magic") from e
+
+    ipython.register_magics(ChooseTypecheckerMagics)