Need cython < 3 right now.

pyproject.toml

@@ -13,8 +13,9 @@ requires = [
      # name to be created so that we can have both foo.py and _foo.so
      # at the same time. 0.29 fixes some issues with Python 3.7,
      # and adds the 3str mode for transition to Python 3. 0.29.14+ is
-     # required for Python 3.8
-     "Cython >= 0.29.14",
+     # required for Python 3.8. As of now, we can't build with Cython 3.0a3
+     # https://github.com/gevent/gevent/issues/1599
+     "Cython >= 0.29.14, < 3",
      # See version requirements in setup.py
      "cffi >= 1.12.3 ; platform_python_implementation == 'CPython'",
      # Python 3.7 requires at least 0.4.14, which is ABI incompatible with earlier