fix(filehandler): Fix local files' opening mode

With the LocalFileHandler, if the "mode" argument did not contain "b"
for binary mode explicitly, files would actually be opened in text mode.
This clashed with the type annotations and documentation.

This commit always explicitly enables binary mode when opening the
underlying file.

capellambse/filehandler/local.py

@@ -40,15 +40,15 @@ def open(
         normpath = helpers.normalize_pure_path(filename)
         if "w" not in mode or self.__transaction is None:
             path = self.path / normpath
-            return t.cast(t.BinaryIO, path.open(mode))
+            return path.open("rb")
 
         if normpath in self.__transaction:
             raise RuntimeError(
                 f"File already written in this transaction: {normpath}"
             )
         self.__transaction.add(normpath)
         tmppath = _tmpname(normpath)
-        return t.cast(t.BinaryIO, (self.path / tmppath).open(mode))
+        return (self.path / tmppath).open("wb")
 
     def get_model_info(self) -> ModelInfo:
         assert isinstance(self.path, pathlib.Path)