move Keithley2600Channel to if TYPE_CHECKING block.

Author: Thomas Lemon

tests/drivers/test_keithley_26xx.py

@@ -1,17 +1,19 @@
 from collections import Counter
-from typing import cast
+from typing import TYPE_CHECKING, cast
 from unittest.mock import patch
 
 import numpy as np
 import pytest
 
 from qcodes.dataset import LinSweep
 from qcodes.instrument_drivers.Keithley import (
-    Keithley2600Channel,
     Keithley2600MeasurementStatus,
     Keithley2614B,
 )
 
+if TYPE_CHECKING:
+    from qcodes.instrument_drivers.Keithley import Keithley2600Channel
+
 
 @pytest.fixture(scope="function", name="driver")
 def _make_driver():