compile_build_api: switch to typing module

Converting Python docstring annotations to typing.

BUG=b:196895668
TEST=./run_tests api/compile_build_api_proto_unittests.py

Change-Id: I7fd9c21b85cb9e0a589a22ccba9c51064c547f3f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3097519
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Tested-by: Miriam Polzer <mpolzer@google.com>
Commit-Queue: Miriam Polzer <mpolzer@google.com>
diff --git a/api/compile_build_api_proto.py b/api/compile_build_api_proto.py
index 01aa0ba..ff6daff 100644
--- a/api/compile_build_api_proto.py
+++ b/api/compile_build_api_proto.py
@@ -90,7 +90,7 @@
     cros_build_lib.run(cmd, cwd=constants.CHROMITE_DIR, print_cmd=False)
 
 
-def _CleanTargetDirectory(directory):
+def _CleanTargetDirectory(directory: str):
   """Remove any existing generated files in the directory.
 
   This clean only removes the generated files to avoid accidentally destroying
@@ -100,7 +100,7 @@
   diagnosing accidental __init__.py changes.
 
   Args:
-    directory (str): Path to be cleaned up.
+    directory: Path to be cleaned up.
   """
   logging.info('Cleaning old files from %s.', directory)
   for dirpath, _dirnames, filenames in os.walk(directory):