Fix mypy type issue (#4745)

Author: cobaltt7

.github/workflows/pypi_upload.yml

@@ -50,7 +50,7 @@ jobs:
       # Keep cibuildwheel version in sync with below
       - name: Install cibuildwheel and pypyp
         run: |
-          pipx install cibuildwheel==2.22.0
+          pipx install cibuildwheel==2.23.3
           pipx install pypyp==1.3.0
       - name: generate matrix
         if: github.event_name != 'pull_request'

.pre-commit-config.yaml

@@ -39,7 +39,7 @@ repos:
         exclude: ^src/blib2to3/
 
   - repo: https://github.com/pre-commit/mirrors-mypy
-    rev: v1.16.1
+    rev: v1.17.1
     hooks:
       - id: mypy
         exclude: ^(docs/conf.py|scripts/generate_schema.py)$

pyproject.toml

@@ -126,7 +126,7 @@ macos-max-compat = true
 enable-by-default = false
 dependencies = [
   "hatch-mypyc>=0.16.0",
-  "mypy>=1.12",
+  "mypy==1.17.1",
   "click>=8.1.7",
 ]
 require-runtime-dependencies = true

src/black/trans.py

@@ -334,6 +334,9 @@ class CustomSplit:
     break_idx: int
 
 
+CustomSplitMapKey = tuple[StringID, str]
+
+
 @trait
 class CustomSplitMapMixin:
     """
@@ -342,13 +345,12 @@ class CustomSplitMapMixin:
     the resultant substrings go over the configured max line length.
     """
 
-    _Key: ClassVar = tuple[StringID, str]
-    _CUSTOM_SPLIT_MAP: ClassVar[dict[_Key, tuple[CustomSplit, ...]]] = defaultdict(
-        tuple
+    _CUSTOM_SPLIT_MAP: ClassVar[dict[CustomSplitMapKey, tuple[CustomSplit, ...]]] = (
+        defaultdict(tuple)
     )
 
     @staticmethod
-    def _get_key(string: str) -> "CustomSplitMapMixin._Key":
+    def _get_key(string: str) -> CustomSplitMapKey:
         """
         Returns:
             A unique identifier that is used internally to map @string to a