Issue #22191: Fix warnings.__all__.

Thanks to Jon Poler for the patch.
diff --git a/Lib/warnings.py b/Lib/warnings.py
index 08b70af..bf9a5d8 100644
--- a/Lib/warnings.py
+++ b/Lib/warnings.py
@@ -7,7 +7,8 @@
 import sys
 import types
 
-__all__ = ["warn", "showwarning", "formatwarning", "filterwarnings",
+__all__ = ["warn", "warn_explicit", "showwarning",
+           "formatwarning", "filterwarnings", "simplefilter",
            "resetwarnings", "catch_warnings"]