fix: config directory may already be rendered string

We determine variant pages late in the game when the variant
config was merged into the document config

Author: dannyadair

src/pdfbaker/config.py

@@ -85,6 +85,9 @@ def resolve_path(self, spec: PathSpec, directory: Path | None = None) -> Path:
             Resolved Path object
         """
         directory = directory or self["directories"]["config"]
+        if isinstance(directory, str):
+            directory = Path(directory)
+
         if isinstance(spec, str):
             return directory / spec