fix: regression

Fix regression introduced where string operations were being attempted on
Path object. Replace the operation with an equivalent Pathlib operation
and then cast back to a string as otherwise the wildcard substitution
fails.

This kind of regression highlights the need for integration tests that
check a representative config can be loaded correctly. Right now it's
quite difficult to refactor without introducing bugs like this with
seemingly innocuous changes.

Signed-off-by: James McCorrie <james.mccorrie@lowrisc.org>

Author: machshev

src/dvsim/flow/base.py

@@ -132,7 +132,9 @@ def __init__(self, flow_cfg_file, hjson_data, args, mk_config) -> None:
                 self._load_child_cfg(entry, mk_config)
 
         if self.rel_path == "":
-            self.rel_path = Path(self.flow_cfg_file).parent.replace(self.proj_root + "/", "")
+            self.rel_path = str(
+                Path(self.flow_cfg_file).parent.relative_to(self.proj_root),
+            )
 
         # Process overrides before substituting wildcards
         self._process_overrides()