gh-120417: Fix "imported but unused" linter warnings (#120461)

Add __all__ to the following modules:
importlib.machinery, importlib.util and xml.sax.

Add also "# noqa: F401" in collections.abc,
subprocess and xml.sax.

* Sort __all__; remove collections.abc.__all__; remove private names

* Add tests

Lib/test/test_sax.py

@@ -16,6 +16,7 @@ from xml.sax.expatreader import create_parser
 from xml.sax.handler import (feature_namespaces, feature_external_ges,
                              LexicalHandler)
 from xml.sax.xmlreader import InputSource, AttributesImpl, AttributesNSImpl
+from xml import sax
 from io import BytesIO, StringIO
 import codecs
 import os.path
@@ -25,7 +26,7 @@ import sys
 from urllib.error import URLError
 import urllib.request
 from test.support import os_helper
-from test.support import findfile
+from test.support import findfile, check__all__
 from test.support.os_helper import FakePath, TESTFN
 
 
@@ -1557,5 +1558,20 @@ class CDATAHandlerTest(unittest.TestCase):
         self.assertEqual(self.char_index, 2)
 
 
+class TestModuleAll(unittest.TestCase):
+    def test_all(self):
+        extra = (
+            'ContentHandler',
+            'ErrorHandler',
+            'InputSource',
+            'SAXException',
+            'SAXNotRecognizedException',
+            'SAXNotSupportedException',
+            'SAXParseException',
+            'SAXReaderNotAvailable',
+        )
+        check__all__(self, sax, extra=extra)
+
+
 if __name__ == "__main__":
     unittest.main()