diff --git a/doc/conf.py b/doc/conf.py
index da753aa9c66..6980eb91595 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -6,14 +6,7 @@
 import time
 from typing import TYPE_CHECKING
 
-from sphinx import __display_version__, addnodes
-from sphinx.application import Sphinx
-from sphinx.environment import BuildEnvironment
-
-if TYPE_CHECKING:
-    from pathlib import Path
-
-    from docutils import nodes
+from sphinx import __display_version__
 
 os.environ['SPHINX_AUTODOC_RELOAD_MODULES'] = '1'
 
@@ -263,10 +256,20 @@
 
 # -- Extension interface -------------------------------------------------------
 
+from sphinx import addnodes  # NoQA: E402
+
+if TYPE_CHECKING:
+    from pathlib import Path
+
+    from docutils.nodes import Element
+
+    from sphinx.application import Sphinx
+    from sphinx.environment import BuildEnvironment
+
 _event_sig_re = re.compile(r'([a-zA-Z-]+)\s*\((.*)\)')
 
 
-def parse_event(_env: BuildEnvironment, sig: str, signode: nodes.Element) -> str:
+def parse_event(_env: BuildEnvironment, sig: str, signode: Element) -> str:
     m = _event_sig_re.match(sig)
     if m is None:
         signode += addnodes.desc_name(sig, sig)
diff --git a/pyproject.toml b/pyproject.toml
index f9202d8e839..9e00b59c2ac 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -137,7 +137,12 @@ exclude = [
 ]
 
 [tool.mypy]
-files = ["sphinx", "utils", "tests", "doc/conf.py"]
+files = [
+    "doc/conf.py",
+    "sphinx",
+    "tests",
+    "utils",
+]
 exclude = [
     "tests/roots",
     # tests/
diff --git a/sphinx/ext/autodoc/__init__.py b/sphinx/ext/autodoc/__init__.py
index 63ec1a0e8f8..4f6e776d656 100644
--- a/sphinx/ext/autodoc/__init__.py
+++ b/sphinx/ext/autodoc/__init__.py
@@ -177,7 +177,7 @@ def merge_members_option(options: dict) -> None:
 
 # Some useful event listener factories for autodoc-process-docstring.
 
-def cut_lines(pre: int, post: int = 0, what: str | list[str] | None = None) -> Callable:
+def cut_lines(pre: int, post: int = 0, what: Sequence[str] | None = None) -> Callable:
     """Return a listener that removes the first *pre* and last *post*
     lines of every docstring.  If *what* is a sequence of strings,
     only docstrings of a type in *what* will be processed.
@@ -185,13 +185,15 @@ def cut_lines(pre: int, post: int = 0, what: str | list[str] | None = None) -> C
     Use like this (e.g. in the ``setup()`` function of :file:`conf.py`)::
 
        from sphinx.ext.autodoc import cut_lines
-       app.connect('autodoc-process-docstring', cut_lines(4, what=['module']))
+       app.connect('autodoc-process-docstring', cut_lines(4, what={'module'}))
 
     This can (and should) be used in place of :confval:`automodule_skip_lines`.
     """
+    what_unique = frozenset(what or ())
+
     def process(app: Sphinx, what_: str, name: str, obj: Any, options: Any, lines: list[str],
                 ) -> None:
-        if what and what_ not in what:
+        if what_ not in what_unique:
             return
         del lines[:pre]
         if post: