Fix --package-root tests for Windows 3.13

A change to relpath in 3.13 seems to have broken
the handling of paths beginning with `\\`.

Author: emmatyping

mypy/fscache.py

@@ -117,9 +117,9 @@ def init_under_package_root(self, path: str) -> bool:
             if not stat.S_ISDIR(st.st_mode):
                 return False
         ok = False
-        drive, path = os.path.splitdrive(path)  # Ignore Windows drive name
         if os.path.isabs(path):
             path = os.path.relpath(path)
+        drive, path = os.path.splitdrive(path)  # Ignore Windows drive name
         path = os.path.normpath(path)
         for root in self.package_root:
             if path.startswith(root):