os: Include posix functions in os.__all__. Closes issue #18554.

Patch by Ronald Oussoren.
diff --git a/Lib/os.py b/Lib/os.py
index 556f592..3bb0f4e 100644
--- a/Lib/os.py
+++ b/Lib/os.py
@@ -61,6 +61,10 @@
     except ImportError:
         pass
 
+    import posix
+    __all__.extend(_get_exports_list(posix))
+    del posix
+
 elif 'nt' in _names:
     name = 'nt'
     linesep = '\r\n'
diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
index 020d0fa..6281596 100644
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -2616,6 +2616,13 @@
         self.assertEqual(os.get_blocking(fd), True)
 
 
+
+class ExportsTests(unittest.TestCase):
+    def test_os_all(self):
+        self.assertIn('open', os.__all__)
+        self.assertIn('walk', os.__all__)
+
+
 @support.reap_threads
 def test_main():
     support.run_unittest(
@@ -2652,6 +2659,7 @@
         FDInheritanceTests,
         Win32JunctionTests,
         BlockingTests,
+        ExportsTests,
     )
 
 if __name__ == "__main__":
diff --git a/Misc/NEWS b/Misc/NEWS
index 9cb9e23..fb018be 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,8 @@
 Core and Builtins
 -----------------
 
+- Issue #18554: os.__all__ includes posix functions.
+
 - Issue #21391: Use os.path.abspath in the shutil module.
 
 - Issue #11471: avoid generating a JUMP_FORWARD instruction at the end of