Go back to pinning Cython < 3.

This is because the memory leak tests failed: https://travis-ci.org/github/gevent/gevent/jobs/683782800

.travis.yml

@@ -174,7 +174,7 @@ jobs:
         # First, the build dependencies (see setup.cfg)
         # so that we don't have to use build isolation and can better use the cache;
         # Note that we can't use -U for cffi and greenlet on PyPy.
-        - &build-gevent-deps pip install -U setuptools wheel twine && pip install -U 'faulthandler; python_version == "2.7" and platform_python_implementation == "CPython"' 'cffi;platform_python_implementation=="CPython"' 'cython>=3.0a4' 'greenlet;platform_python_implementation=="CPython"'
+        - &build-gevent-deps pip install -U setuptools wheel twine && pip install -U 'faulthandler; python_version == "2.7" and platform_python_implementation == "CPython"' 'cffi;platform_python_implementation=="CPython"' 'cython<3' 'greenlet;platform_python_implementation=="CPython"'
         # Next, build the wheel *in place*. This helps ccache, and also lets us cache the configure
         # output (pip install uses a random temporary directory, making this difficult)
         - python setup.py bdist_wheel

appveyor.yml

@@ -172,7 +172,7 @@ build_script:
   # Build the compiled extension
   # Try to get some things that don't wind up in the pip cache as
   # built wheels if they're built during an isolated build.
-  - "%CMD_IN_ENV% %PYEXE% -m pip install -U --pre wheel cython setuptools cffi"
+  - "%CMD_IN_ENV% %PYEXE% -m pip install -U wheel cython setuptools cffi"
   - if not "%GWHEEL_ONLY%"=="true" %PYEXE% -m pip install -U -e .[test]
 
 test_script:

docs/changes/1599.bugfix

@@ -2,3 +2,6 @@ gevent can now be built using Cython 3.0a2 and newer.
 
 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
+gevent's test for memory leaks failing.

pyproject.toml

@@ -13,8 +13,11 @@ 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 >= 3.0a4",
+     # 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",
      # 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