Fix Python 2's traceback.format_exception, traceback.print_exception (#1512)

E.g. it's legal to call traceback.format_exception(None, None, None). In
particular, this change makes the following idiom type-check:
  import traceback
  import sys
  exc_type, exc_value, exc_traceback = sys.exc_info()
  traceback.format_exception(exc_type, exc_value, exc_traceback)

Author: matthiaskramm

stdlib/2and3/traceback.pyi

@@ -19,7 +19,8 @@ if sys.version_info >= (3,):
     def print_last(limit: Optional[int] = ..., file: Optional[IO[str]] = ...,
                    chain: bool = ...) -> None: ...
 else:
-    def print_exception(etype: Type[BaseException], value: BaseException,
+    def print_exception(etype: Optional[Type[BaseException]],
+                        value: Optional[BaseException],
                         tb: Optional[TracebackType], limit: Optional[int] = ...,
                         file: Optional[IO[str]] = ...) -> None: ...
     def print_exc(limit: Optional[int] = ...,
@@ -47,8 +48,9 @@ if sys.version_info >= (3,):
                          chain: bool = ...) -> List[str]: ...
     def format_exc(limit: Optional[int] = ..., chain: bool = ...) -> str: ...
 else:
-    def format_exception(etype: Type[BaseException], value: BaseException,
-                         tb: TracebackType,
+    def format_exception(etype: Optional[Type[BaseException]],
+                         value: Optional[BaseException],
+                         tb: Optional[TracebackType],
                          limit: Optional[int] = ...) -> List[str]: ...
     def format_exc(limit: Optional[int] = ...) -> str: ...
 def format_tb(tb: Optional[TracebackType], limit: Optional[int] = ...) -> List[str]: ...