Add missing TraverserVisitor import

Author: madelyngamble2

mypy/checker.py

@@ -149,6 +149,7 @@
 from mypy.options import PRECISE_TUPLE_TYPES, Options
 from mypy.patterns import AsPattern, StarredPattern
 from mypy.traverser import TraverserVisitor
+from mypy.traverser import TraverserVisitor
 from mypy.plugin import Plugin
 from mypy.plugins import dataclasses as dataclasses_plugin
 from mypy.scope import Scope