Add type stub for sys._current_exceptions (#14937)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

Author: guoci

stdlib/sys/__init__.pyi

@@ -354,6 +354,13 @@ else:
 def _current_frames() -> dict[int, FrameType]: ...
 def _getframe(depth: int = 0, /) -> FrameType: ...
 
+# documented -- see https://docs.python.org/3/library/sys.html#sys._current_exceptions
+if sys.version_info >= (3, 12):
+    def _current_exceptions() -> dict[int, BaseException | None]: ...
+
+else:
+    def _current_exceptions() -> dict[int, OptExcInfo]: ...
+
 if sys.version_info >= (3, 12):
     def _getframemodulename(depth: int = 0) -> str | None: ...