syscalls: add typing information

Change-Id: Ibe479360f39dd70d91d2ee9ac64ca86ac6c0d403
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/docs/+/3369304
Tested-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Ram Chandrasekar <rchandrasekar@google.com>
Commit-Queue: Ram Chandrasekar <rchandrasekar@google.com>
diff --git a/constants/constants.py b/constants/constants.py
index a47b5b8..a1cdb80 100644
--- a/constants/constants.py
+++ b/constants/constants.py
@@ -20,7 +20,7 @@
 )
 
 
-def math_eval(expr):
+def math_eval(expr: str) -> str:
     """Evaluate a arithmetic expression."""
     # Only bother listing operators that actually get used.
     operators = {ast.Add: operator.add}