Fix type hint for msgctxt parameter in polib (python#12506)

stubs/polib/polib.pyi

@@ -1,5 +1,5 @@
 from collections.abc import Callable
-from typing import IO, Any, Generic, SupportsIndex, TypeVar, overload
+from typing import IO, Any, Generic, Literal, SupportsIndex, TypeVar, overload
 
 _TB = TypeVar("_TB", bound=_BaseEntry)
 _TP = TypeVar("_TP", bound=POFile)
@@ -38,7 +38,9 @@ class _BaseFile(list[_TB]):
     def insert(self, index: SupportsIndex, entry: _TB) -> None: ...
     def metadata_as_entry(self) -> POEntry: ...
     def save(self, fpath: str | None = ..., repr_method: str = ..., newline: str | None = ...) -> None: ...
-    def find(self, st: str, by: str = ..., include_obsolete_entries: bool = ..., msgctxt: bool = ...) -> _TB | None: ...
+    def find(
+        self, st: str, by: str = ..., include_obsolete_entries: bool = ..., msgctxt: str | Literal[False] = ...
+    ) -> _TB | None: ...
     def ordered_metadata(self) -> list[tuple[str, str]]: ...
     def to_binary(self) -> bytes: ...