Issue #22191: Fix warnings.__all__.

Thanks to Jon Poler for the patch.
diff --git a/Lib/test/test___all__.py b/Lib/test/test___all__.py
index 2c5dbef..789f9fb 100644
--- a/Lib/test/test___all__.py
+++ b/Lib/test/test___all__.py
@@ -73,7 +73,7 @@
             # than an AttributeError somewhere deep in CGIHTTPServer.
             import _socket
 
-        # rlcompleter needs special consideration; it import readline which
+        # rlcompleter needs special consideration; it imports readline which
         # initializes GNU readline which calls setlocale(LC_CTYPE, "")... :-(
         try:
             import rlcompleter
diff --git a/Lib/test/test_warnings.py b/Lib/test/test_warnings.py
index a770ba2..fb2046e 100644
--- a/Lib/test/test_warnings.py
+++ b/Lib/test/test_warnings.py
@@ -62,6 +62,25 @@
         sys.modules['warnings'] = original_warnings
         super(BaseTest, self).tearDown()
 
+class PublicAPITests(BaseTest):
+
+    """Ensures that the correct values are exposed in the
+    public API.
+    """
+
+    def test_module_all_attribute(self):
+        self.assertTrue(hasattr(self.module, '__all__'))
+        target_api = ["warn", "warn_explicit", "showwarning",
+                      "formatwarning", "filterwarnings", "simplefilter",
+                      "resetwarnings", "catch_warnings"]
+        self.assertSetEqual(set(self.module.__all__),
+                            set(target_api))
+
+class CPublicAPITests(PublicAPITests, unittest.TestCase):
+    module = c_warnings
+
+class PyPublicAPITests(PublicAPITests, unittest.TestCase):
+    module = py_warnings
 
 class FilterTests(object):
 
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"]
 
 
diff --git a/Misc/ACKS b/Misc/ACKS
index 4ebe074..f9a4426 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1052,6 +1052,7 @@
 Jean-François Piéronne
 Oleg Plakhotnyuk
 Remi Pointel
+Jon Poler
 Ariel Poliak
 Guilherme Polo
 Illia Polosukhin
diff --git a/Misc/NEWS b/Misc/NEWS
index 47ea776..cbe93b1 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -19,6 +19,8 @@
 Library
 -------
 
+- Issue #22191: Fixed warnings.__all__.
+
 - Issue #21308: Backport numerous features from Python's ssl module. This is
   part of PEP 466.