dependency: switch to typing module

Converting Python docstring annotations to typing.

BUG=b:196895668
TEST=./run_tests api/controller/dependency_unittest.py

Change-Id: I1c88faa7ee19716c529ab3edc6f2a00dcea5d66a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3106806
Reviewed-by: Eric Lin <ericth@google.com>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
Tested-by: Eric Lin <ericth@google.com>
Auto-Submit: Eric Lin <ericth@google.com>
Commit-Queue: Chris McDonald <cjmcdonald@chromium.org>
diff --git a/api/controller/dependency.py b/api/controller/dependency.py
index 659139c..cd8ed63 100644
--- a/api/controller/dependency.py
+++ b/api/controller/dependency.py
@@ -59,13 +59,16 @@
 @faux.empty_error
 @validate.require_each('packages', ['category', 'package_name'])
 @validate.validation_complete
-def GetBuildDependencyGraph(input_proto, output_proto, _config):
+def GetBuildDependencyGraph(
+    input_proto: depgraph_pb2.GetBuildDependencyGraphRequest,
+    output_proto: depgraph_pb2.GetBuildDependencyGraphResponse,
+    _config: api_config.ApiConfig) -> None:
   """Create the build dependency graph.
 
   Args:
-    input_proto (GetBuildDependencyGraphRequest): The input arguments message.
-    output_proto (GetBuildDependencyGraphResponse): The empty output message.
-    _config (api_config.ApiConfig): The API call config.
+    input_proto: The input arguments message.
+    output_proto: The empty output message.
+    _config: The API call config.
   """
   if input_proto.HasField('sysroot'):
     board = input_proto.sysroot.build_target.name