tox: -py36,py37d +py38d, py38

tox.ini

 [tox]
 envlist =
-    {py27d,py27,py36,py37d,py37,pypy,pypy3}-{thread,gevent}
+    {py27d,py27,py37,py38d,py38,pypy,pypy3}-{thread,gevent}
 
 
 # ThreadSanitizer
@@ -10,26 +10,26 @@ envlist =
 # (*) PyPy locks its GIL (see RPyGilAcquire) by manually doing atomic cmpxchg
 # and other games, which TSAN cannot see if PyPy itself was not compiled with
 # -fsanitize=thread.
-    {py27d,py27,py36,py37d,py37           }-{thread       }-tsan
+    {py27d,py27,py37,py38d,py38           }-{thread       }-tsan
 # XXX py*-gevent-tsan would be nice to have, but at present TSAN is not
 # effective with gevent, because it does not understand greenlet "thread"
 # switching and so perceives the program as having only one thread where races
 # are impossible. Disabled to save time.
-#   {py27d,py27,py36,py37d,py37           }-{       gevent}-tsan
+#   {py27d,py27,py37,py38d,py38           }-{       gevent}-tsan
 
 
 # AddressSanitizer
 
 # XXX asan does not work with gevent: https://github.com/python-greenlet/greenlet/issues/113
-    {py27d,py27,py36,py37d,py37,pypy,pypy3}-{thread       }-asan
+    {py27d,py27,py37,py38d,py38,pypy,pypy3}-{thread       }-asan
 
 [testenv]
 basepython =
     py27d:  python2.7-dbg
     py27:   python2.7
-    py36:   python3.6
-    py37d:  python3.7-dbg
     py37:   python3.7
+    py38d:  python3.8-dbg
+    py38:   python3.8
     pypy:   pypy
     pypy3:  pypy3