diff --git a/src/flake8/options/manager.py b/src/flake8/options/manager.py
index 5e0fee2..f537a42 100644
--- a/src/flake8/options/manager.py
+++ b/src/flake8/options/manager.py
@@ -82,11 +82,11 @@ def _flake8_normalize(value, *args, **kwargs):
         raise TypeError(f"Unexpected keyword args: {kwargs}")
 
     ret = value  # type: Union[str, List[str]]
-    if comma_separated_list and isinstance(ret, utils.string_types):
+    if comma_separated_list and isinstance(ret, str):
         ret = utils.parse_comma_separated_list(value)
 
     if normalize_paths:
-        if isinstance(ret, utils.string_types):
+        if isinstance(ret, str):
             ret = utils.normalize_path(ret, *args)
         else:
             ret = utils.normalize_paths(ret, *args)
@@ -212,7 +212,7 @@ class Option:
                 nargs = 0
 
         # optparse -> argparse for `type`
-        if isinstance(type, utils.string_types):
+        if isinstance(type, str):
             LOG.warning(
                 "option %s: please update from optparse string `type=` to "
                 "argparse callable `type=` -- this will be an error in the "
@@ -299,9 +299,7 @@ class Option:
     def normalize(self, value, *normalize_args):
         # type: (Any, *str) -> Any
         """Normalize the value based on the option configuration."""
-        if self.comma_separated_list and isinstance(
-            value, utils.string_types
-        ):
+        if self.comma_separated_list and isinstance(value, str):
             value = utils.parse_comma_separated_list(value)
 
         if self.normalize_paths:
diff --git a/src/flake8/utils.py b/src/flake8/utils.py
index 074cf0b..6bf0b49 100644
--- a/src/flake8/utils.py
+++ b/src/flake8/utils.py
@@ -29,7 +29,6 @@ if False:  # `typing.TYPE_CHECKING` was introduced in 3.5.2
 DIFF_HUNK_REGEXP = re.compile(r"^@@ -\d+(?:,\d+)? \+(\d+)(?:,(\d+))? @@.*$")
 COMMA_SEPARATED_LIST_RE = re.compile(r"[,\s]")
 LOCAL_PLUGIN_LIST_RE = re.compile(r"[,\t\n\r\f\v]")
-string_types = (str, type(""))
 
 
 def parse_comma_separated_list(value, regexp=COMMA_SEPARATED_LIST_RE):
@@ -48,7 +47,7 @@ def parse_comma_separated_list(value, regexp=COMMA_SEPARATED_LIST_RE):
     :rtype:
         list
     """
-    assert isinstance(value, string_types), value
+    assert isinstance(value, str), value
 
     separated = regexp.split(value)
     item_gen = (item.strip() for item in separated)
@@ -96,7 +95,7 @@ def parse_files_to_codes_mapping(value_):  # noqa: C901
     :param value: String to be parsed and normalized.
     :type value: str
     """
-    if not isinstance(value_, string_types):
+    if not isinstance(value_, str):
         value = "\n".join(value_)
     else:
         value = value_