Account for file directive with string in pyproject.toml

setuptools/config/pyprojecttoml.py

@@ -11,6 +11,7 @@
 from typing import TYPE_CHECKING, Callable, Dict, Optional, Mapping, Set, Union
 
 from setuptools.errors import FileError, OptionError
+from setuptools.extern.more_itertools import always_iterable
 
 from . import expand as _expand
 from ._apply_pyprojecttoml import apply as _apply
@@ -312,8 +313,9 @@ def _expand_directive(
         with _ignore_errors(self.ignore_option_errors):
             root_dir = self.root_dir
             if "file" in directive:
-                self._referenced_files.update(directive["file"])
-                return _expand.read_files(directive["file"], root_dir)
+                files = always_iterable(directive["file"])
+                self._referenced_files.update(files)
+                return _expand.read_files(files, root_dir)
             if "attr" in directive:
                 return _expand.read_attr(directive["attr"], package_dir, root_dir)
             raise ValueError(f"invalid `{specifier}`: {directive!r}")