Ghostsciptbackend flake8 fixes

Author: bosd

camelot/backends/ghostscript_backend.py

@@ -29,7 +29,7 @@ def convert(self, pdf_path: str, png_path: str, resolution: int = 300) -> None:
             raise OSError(
                 "Ghostscript is not installed. You can install it using the instructions"
                 " here: https://pypdf-table-extraction.readthedocs.io/en/latest/user/install-deps.html"
-            )
+            ) from ex
 
         gs_command = [
             "gs",