plantuml: fix function wrapper in tests not to use deprecated func_name

tests/test_functional.py

@@ -52,7 +52,7 @@ def test():
             finally:
                 os.unlink(os.path.join(_srcdir, 'index.rst'))
                 shutil.rmtree(_outdir)
-        test.func_name = func.func_name
+        test.__name__ = func.__name__
         return test
     return wrapfunc