Add missing msg type to ImportError (#5111)

Fixes #5107

stdlib/builtins.pyi

@@ -1311,6 +1311,7 @@ class ImportError(_StandardError):
     def __init__(self, *args: object, name: Optional[str] = ..., path: Optional[str] = ...) -> None: ...
     name: Optional[str]
     path: Optional[str]
+    msg: str  # undocumented
 
 class LookupError(_StandardError): ...
 class MemoryError(_StandardError): ...