[pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

mypy/checker.py

@@ -41,7 +41,6 @@
 from mypy.errorcodes import TYPE_VAR, UNUSED_AWAITABLE, UNUSED_COROUTINE, ErrorCode
 from mypy.errors import Errors, ErrorWatcher, report_internal_error
 from mypy.expandtype import expand_self_type, expand_type, expand_type_by_instance
-from mypy.join import join_types
 from mypy.literals import Key, extract_var_from_literal_hash, literal, literal_hash
 from mypy.maptype import map_instance_to_supertype
 from mypy.meet import is_overlapping_erased_types, is_overlapping_types