mesonlib: Fix bug in relpath return values found by mypy

mesonbuild/utils/universal.py

@@ -1930,14 +1930,14 @@ class OrderedSet(T.MutableSet[_T]):
         for item in iterable:
             self.discard(item)
 
-def relpath(path: str, start: str) -> str:
+def relpath(path: T.Union[str, Path], start: T.Union[str, Path]) -> str:
     # On Windows a relative path can't be evaluated for paths on two different
     # drives (i.e. c:\foo and f:\bar).  The only thing left to do is to use the
     # original absolute path.
     try:
         return os.path.relpath(path, start)
     except (TypeError, ValueError):
-        return path
+        return str(path)
 
 def path_is_in_root(path: Path, root: Path, resolve: bool = False) -> bool:
     # Check whether a path is within the root directory root