Update release date for Python 3.6 [skip ci]

doc/whatsnew_1_2.rst

@@ -24,7 +24,7 @@ gevent 1.2 supports Python 2.7, 3.4, 3.5 and 3.6 on the CPython
 .. note:: PyPy is not supported on Windows. (gevent's CFFI backend is not
          available on Windows.)
 
-Python 3.6 is currently in beta and is supported at the same level as 3.5.
+Python 3.6 was released recently and is supported at the same level as 3.5.
 
 For ease of installation on Windows and OS X, gevent 1.2 is
 distributed as pre-compiled binary wheels, in addition to source code.