Bump min Cython for 3.10; try to install 3.10 on appveyor.

.github/workflows/ci.yml

@@ -257,7 +257,7 @@ jobs:
           pip install -U -q setuptools wheel twine
           pip install -q -U 'faulthandler; python_version == "2.7" and platform_python_implementation == "CPython"'
           pip install -q -U 'cffi;platform_python_implementation=="CPython"'
-          pip install -q -U 'cython>=3.0a5'
+          pip install -q -U 'cython>=3.0a9'
           pip install 'greenlet>=1.0a1;platform_python_implementation=="CPython"'
 
       - name: Build gevent

appveyor.yml

@@ -41,6 +41,12 @@ environment:
     # a later point release.
 
     # 64-bit
+    - PYTHON: "C:\\Python310-x64"
+      PYTHON_VERSION: "3.10.0rc1"
+      PYTHON_ARCH: "64"
+      PYTHON_EXE: python
+      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
+
 
     - PYTHON: "C:\\Python39-x64"
       PYTHON_VERSION: "3.9.x"

pyproject.toml

@@ -20,8 +20,9 @@ requires = [
      # 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);
      # This was fixed in 3.0a5 (https://github.com/cython/cython/issues/3578)
-     # 3.0a6 fixes an issue cythonizing source on 32-bit platforms
-     "Cython >= 3.0a8",
+     # 3.0a6 fixes an issue cythonizing source on 32-bit platforms.
+     # 3.0a9 is needed for Python 3.10.
+     "Cython >= 3.0a9",
      # 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