Rename variable to avoid mypy type error (#23669)

Reusing variables can cause a mypy type error (python/mypy#1174).
This previously went unnoticed because the previous and current use of `key` was `str`, but in
#23644 the previous use was retyped to `Text`
and as such caused a unicode-vs-str type error in mypy.

tools/manifest/sourcefile.py

@@ -992,9 +992,9 @@ def manifest_items(self):
 
         if drop_cached and "__cached_properties__" in self.__dict__:
             cached_properties = self.__dict__["__cached_properties__"]
-            for key in cached_properties:
-                if str(key) in self.__dict__:
-                    del self.__dict__[str(key)]
+            for prop in cached_properties:
+                if prop in self.__dict__:
+                    del self.__dict__[prop]
             del self.__dict__["__cached_properties__"]
 
         return rv