Move the __weakref__ to the base class for Cython because we did see a failure...

Move the __weakref__ to the base class for Cython because we did see a failure in 3.6 too: https://travis-ci.org/gevent/gevent/jobs/379718754#L753

CHANGES.rst

@@ -8,8 +8,7 @@
 ==================
 
 - Fix weak references to :class:`gevent.event.Event`. Reported in
-  :issue:`1211` by Matias Guijarro. Note that creating weak references
-  to Event objects may crash Python 3.7.
+  :issue:`1211` by Matias Guijarro.
 
 
 1.3.0 (2018-05-11)

src/gevent/_event.pxd

@@ -36,6 +36,11 @@ cdef inline void greenlet_init():
 cdef void _init()
 
 cdef class _AbstractLinkable:
+   # We declare the __weakref__ here in the base (even though
+   # that's not really what we want) as a workaround for a Cython
+   # issue we see reliably on 3.7b4 and sometimes on 3.6. See
+   # https://github.com/cython/cython/issues/2270
+   cdef object __weakref__
    cdef _notifier
    cdef set _links
    cdef readonly SwitchOutGreenletWithLoop hub
@@ -53,7 +58,6 @@ cdef class _AbstractLinkable:
 
 
 cdef class Event(_AbstractLinkable):
-   cdef object __weakref__
    cdef bint _flag
 
 cdef class AsyncResult(_AbstractLinkable):

src/greentest/test__event.py

@@ -238,13 +238,13 @@ class TestWait_count2(TestWait):
 
 class TestEventBasics(greentest.TestCase):
 
-    @greentest.skipOnPy37("Cython crashes in this case. "
-                          "See https://github.com/cython/cython/issues/2270")
     def test_weakref(self):
         # Event objects should allow weakrefs
         e = Event()
         r = weakref.ref(e)
         self.assertIs(e, r())
+        del e
+        del r
 
 
 del AbstractGenericGetTestCase