bpo-32969: Expose some missing constants in zlib and fix the doc (GH-5988)

(cherry picked from commit bc3f2289b9007396bfb7f986bee477b6176c1822)

Co-authored-by: Xiang Zhang <angwerzx@126.com>
diff --git a/Lib/test/test_zlib.py b/Lib/test/test_zlib.py
index db950fc..99aa89b 100644
--- a/Lib/test/test_zlib.py
+++ b/Lib/test/test_zlib.py
@@ -434,7 +434,8 @@
     def test_flushes(self):
         # Test flush() with the various options, using all the
         # different levels in order to provide more variations.
-        sync_opt = ['Z_NO_FLUSH', 'Z_SYNC_FLUSH', 'Z_FULL_FLUSH']
+        sync_opt = ['Z_NO_FLUSH', 'Z_SYNC_FLUSH', 'Z_FULL_FLUSH',
+                    'Z_PARTIAL_FLUSH', 'Z_BLOCK']
         sync_opt = [getattr(zlib, opt) for opt in sync_opt
                     if hasattr(zlib, opt)]
         data = HAMLET_SCENE * 8