[traceback] Expose print_list method in traceback for python3. (#2837)

stdlib/2and3/traceback.pyi

@@ -1,6 +1,6 @@
 # Stubs for traceback
 
-from typing import Any, Dict, Generator, IO, Iterator, List, Mapping, Optional, Tuple, Type, Iterable
+from typing import Any, Dict, Generator, IO, Iterator, List, Mapping, Optional, Protocol, Tuple, Type, Iterable
 from types import FrameType, TracebackType
 import sys
 
@@ -36,6 +36,10 @@ if sys.version_info >= (3, 5):
     def extract_stack(f: Optional[FrameType] = ...,
                       limit: Optional[int] = ...) -> StackSummary: ...
     def format_list(extracted_list: List[FrameSummary]) -> List[str]: ...
+    class _Writer(Protocol):
+        def write(self, s: str) -> Any: ...
+    # undocumented
+    def print_list(extracted_list: List[FrameSummary], file: Optional[_Writer] = ...) -> None: ...
 else:
     def extract_tb(tb: Optional[TracebackType], limit: Optional[int] = ...) -> List[_PT]: ...
     def extract_stack(f: Optional[FrameType] = ...,