Update to Cython 3.0a5. Fixes #1599

docs/changes/1599.bugfix

-gevent can now be built using Cython 3.0a2 and newer.
+gevent can now be built using Cython 3.0a5 and newer. The PyPI
+distribution uses this version.
 
 The libev extension was incompatible with this. As part of this,
 certain internal, undocumented names have been changed.
 
-However, up through at least 3.0a4 compiling with Cython 3 results in
+(Technically, gevent can be built with Cython 3.0a2 and above.
+However, up through 3.0a4 compiling with Cython 3 results in
 gevent's test for memory leaks failing. See `this Cython issue
-<https://github.com/cython/cython/issues/3578>`_.
+<https://github.com/cython/cython/issues/3578>`_.)

pyproject.toml

@@ -16,8 +16,9 @@ requires = [
      # required for Python 3.8. 3.0a2 introduced a change that prevented
      # us from compiling (https://github.com/gevent/gevent/issues/1599)
      # but once that was fixed, 3.0a4 led to all of our leak tests
-     # failing in Python 2 (https://travis-ci.org/github/gevent/gevent/jobs/683782800)
-     "Cython >= 0.29.14, < 3",
+     # failing in Python 2 (https://travis-ci.org/github/gevent/gevent/jobs/683782800);
+     # This was fixed in 3.0a5 (https://github.com/cython/cython/issues/3578)
+     "Cython >= 3.0a5",
      # 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