BUG: Fix mypy error

Define ip only if we're type checking.

IPython/core/tests/test_async_helpers.py

@@ -9,6 +9,12 b' from textwrap import dedent, indent'
 from unittest import TestCase
 from IPython.testing.decorators import skip_without
 import sys
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from IPython import get_ipython
+    ip = get_ipython()
+
 
 iprc = lambda x: ip.run_cell(dedent(x)).raise_error()
 iprc_nr = lambda x: ip.run_cell(dedent(x))