Fix Python warning in tag_versions.py script

Change-Id: If02301c9d08ab45e2982112f053fbd9277412a58
diff --git a/tools/tag_versions.py b/tools/tag_versions.py
index cdedf53..bcaf056 100755
--- a/tools/tag_versions.py
+++ b/tools/tag_versions.py
@@ -15,7 +15,7 @@
 import utils
 
 # Grep match string for 'Version X.Y.Z[-dev]'
-VERSION_EXP = '^Version [[:digit:]]\+.[[:digit:]]\+.[[:digit:]]\+\(\|-dev\)$'
+VERSION_EXP = r'^Version [[:digit:]]\+.[[:digit:]]\+.[[:digit:]]\+\(\|-dev\)$'
 
 # R8 is located in the 'builder' library.
 AGP_MAVEN = "https://dl.google.com/android/maven2/com/android/tools/build/builder"