fix(dataclasses): Fix type issue with SkipDefaultFieldsReprMixin

Author: tony

src/libtmux/_internal/dataclasses.py

@@ -5,8 +5,12 @@
 This is an internal API not covered by versioning policy.
 """
 import dataclasses
+import typing as t
 from operator import attrgetter
 
+if t.TYPE_CHECKING:
+    from _typeshed import DataclassInstance
+
 
 class SkipDefaultFieldsReprMixin:
     r"""Skip default fields in :func:`~dataclasses.dataclass`
@@ -72,7 +76,7 @@ class SkipDefaultFieldsReprMixin:
     ItemWithMixin(name=Test, unit_price=2.05)
     """
 
-    def __repr__(self) -> str:
+    def __repr__(self: "DataclassInstance") -> str:
         """Omit default fields in object representation."""
         nodef_f_vals = (
             (f.name, attrgetter(f.name)(self))