[mypyc] Add type annotation to setup.py (#20311)

I was getting mypy errors from setup.py when running `python runtests.py
self`.

Author: JukkaL

mypyc/lib-rt/setup.py

@@ -99,7 +99,7 @@ def run(self) -> None:
     kwargs: dict[str, Any]
     if sys.platform == "darwin":
         kwargs = {"language": "c++"}
-        compile_args = []
+        compile_args: list[str] = []
     else:
         kwargs = {}
         compile_args = ["--std=c++11"]