lib: convert SOURCE_ROOT to Path

BUG=None
TEST=CQ passes

Change-Id: I5a61baba4e8236b71771fbaf540c9631253aa88d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4766249
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Cindy Lin <xcl@google.com>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Cindy Lin <xcl@google.com>
diff --git a/api/controller/copybot.py b/api/controller/copybot.py
index fe2660c..ee39098 100644
--- a/api/controller/copybot.py
+++ b/api/controller/copybot.py
@@ -33,8 +33,7 @@
     """Run copybot. Translate all fields in the input protobuf to CLI args."""
 
     cmd = [
-        Path(constants.SOURCE_ROOT)
-        / "src/platform/dev/contrib/copybot/copybot.py"
+        constants.SOURCE_ROOT / "src/platform/dev/contrib/copybot/copybot.py"
     ]
 
     if input_proto.topic: