MNT: Remove py3.9 pathlib handling

lib/cartopy/io/img_nest.py

@@ -122,7 +122,15 @@ def world_files(fname):
             fext_types.extend([ext.upper() for ext in fext_types])
             result = [path.with_suffix(ext) for ext in fext_types]
 
-        result += [path.with_stem(path.stem.swapcase()) for path in result]
+        def _convert_basename(path):
+            # TODO: Once we support only Python 3.9+ we can inline this
+            #       with path.with_stem(path.stem.swapcase())
+            dirname, basename = path.parent, Path(path.name)
+            base, ext = basename.stem, basename.suffix
+            result = dirname / (base.swapcase() + ext)
+            return result
+
+        result += [_convert_basename(path) for path in result]
         # return string paths rather than Path objects
         return [str(r) for r in result]