Add explicit definition for __repr__ and __str__ in BaseException

Taint analysis tools such as Pysa might want to mark `BaseException.__str__`
and `BaseException.__repr__` as a source. However, since those methods
are inherited, this is not possible. Making the definition explicit fixes
the problem.

Author: arthaud

stdlib/builtins.pyi

@@ -2058,6 +2058,10 @@ class BaseException:
     def __new__(cls, *args: Any, **kwds: Any) -> Self: ...
     def __setstate__(self, state: dict[str, Any] | None, /) -> None: ...
     def with_traceback(self, tb: TracebackType | None, /) -> Self: ...
+    # Necessary for security-focused static analyzers (e.g, pysa)
+    # See https://github.com/python/typeshed/pull/14900
+    def __str__(self) -> str: ...  # noqa: Y029
+    def __repr__(self) -> str: ...  # noqa: Y029
     if sys.version_info >= (3, 11):
         # only present after add_note() is called
         __notes__: list[str]