Issue #22351: Fix test_nntplib if the ssl module is missing

@unittest.skipUnless(ssl, '...') doesn't work because the class body uses the
nntplib.NNTP_SSL attribute which doesn't exist.

Lib/test/test_nntplib.py

@@ -1509,8 +1509,8 @@ class MockSocketTests(unittest.TestCase):
             Handler, nntplib.NNTPPermanentError, authinfo_response,
             login, password)
 
-@unittest.skipUnless(ssl, 'requires SSL support')
-class MockSslTests(MockSocketTests):
+if ssl is not None:
+    class MockSslTests(MockSocketTests):
         class nntp_class(nntplib.NNTP_SSL):
             def __init__(self, *pos, **kw):
                 class bypass_context:
@@ -1519,5 +1519,6 @@ class MockSslTests(MockSocketTests):
                         return sock
                 return super().__init__(*pos, ssl_context=bypass_context, **kw)
 
+
 if __name__ == "__main__":
     unittest.main()