chore(formats): simplify PoHeaderMixin typing

Avoid making it generic, but rather type limit the store.

Author: nijel

weblate/formats/ttkit.py

@@ -118,6 +118,7 @@
 PO_DOCSTRING_LOCATION = re.compile(r":docstring of [a-zA-Z0-9._]+:[0-9]+")
 XLIFF_FUZZY_STATES = {"new", "needs-translation", "needs-adaptation", "needs-l10n"}
 _CSV_MAX_PLURAL_FORMS = 100
+type PoHeaderUnit = pounit | PoXliffUnit
 
 
 class CSVMetadataError(ValueError):
@@ -1542,15 +1543,15 @@ def set_state(self, state) -> None:
             self.unit.marktranslatable(False)
 
 
-class PoHeaderMixin[
-    S: pofile | PoXliffFile,
-    U: pounit | PoXliffUnit,
-    T: TTKitUnit,
-](TTKitFormat[S, U, T]):
+class PoHeaderMixin:
+    store: pofile | PoXliffFile
+
     def _po_header_store(self) -> poheader:
-        return self.store
+        return cast("poheader", self.store)
 
-    def _ensure_po_header_first(self, header: U | None = None) -> U | None:
+    def _ensure_po_header_first(
+        self, header: PoHeaderUnit | None = None
+    ) -> PoHeaderUnit | None:
         """Keep PO-style header in the position expected by Translate Toolkit."""
         units = self.store.units
         if header is None:
@@ -1640,7 +1641,9 @@ def update_header(self, file_format_params: FileFormatParams, **kwargs) -> None:
         self._ensure_po_header_first(store.updateheader(add=True, **kwargs))
 
 
-class BasePoFormat[S: pofile, U: pounit, T: BasePoUnit](PoHeaderMixin[S, U, T]):
+class BasePoFormat[S: pofile, U: pounit, T: BasePoUnit](
+    PoHeaderMixin, TTKitFormat[S, U, T]
+):
     loader = pofile  # type: ignore[assignment]
     plural_preference: tuple[int, ...] | None = None
     supports_plural: bool = True
@@ -1832,10 +1835,7 @@ class RichXliffFormat(XliffFormat):
     unit_class = RichXliffUnit
 
 
-class PoXliffFormat(
-    PoHeaderMixin[PoXliffFile, PoXliffUnit, XliffUnit[PoXliffUnit, XliffFormat]],
-    XliffFormat,
-):
+class PoXliffFormat(PoHeaderMixin, XliffFormat):
     # Translators: File format name
     name = gettext_lazy("XLIFF 1.2 with gettext extensions")
     format_id = "poxliff"