Apply patch for Python 3.8 on TestFunctionalMapDataPipe.test_docstring

Same as 52be9e1  but for MapDataPipe

test/test_datapipe.py

@@ -1948,7 +1948,14 @@ def test_docstring(self):
             "shuffle",
             "zip",
         ]:
-            docstring = pydoc.render_doc(thing=getattr(input_dp, dp_funcname))
+            if sys.version_info >= (3, 9):
+                docstring = pydoc.render_doc(
+                    thing=getattr(input_dp, dp_funcname), forceload=True
+                )
+            elif sys.version_info < (3, 9):
+                # pydoc works differently on Python 3.8, see
+                # https://docs.python.org/3/whatsnew/3.9.html#pydoc
+                docstring = getattr(input_dp, dp_funcname).__doc__
             assert f"(functional name: ``{dp_funcname}``)" in docstring
             assert "Args:" in docstring
             assert "Example:" in docstring or "Examples:" in docstring