diff --git a/setup.py b/setup.py
index feed325ca..b0b380354 100644
--- a/setup.py
+++ b/setup.py
@@ -225,9 +225,7 @@ def lib_ext_kwargs(pc, prefix_env_var, lib_name, lib_pkg_name, pc_version, lib_s
 
     if cythonize and cythonizing:
         cython_opts = dict(
-            # default language_level will be '3str' starting from Cython 3.0.0,
-            # but old cython versions (< 0.29) do not know that, thus we use 3 for now.
-            compiler_directives={'language_level': 3},
+            compiler_directives={'language_level': '3str'},
         )
         if not is_win32:
             # compile .pyx extensions to .c in parallel, does not work on windows