tox -= Python3.6

Debian dropped Python3.6 from python3-distutils. Without it I'm no
longer being able to run tests on that python version:

	(neo) (z-dev) (g.env) kirr@deco:~/src/tools/go/pygolang$ tox -e py36-thread
	execution failed:  -- Traceback (most recent call last):
	  File "<string>", line 1, in <module>
	ModuleNotFoundError: No module named 'distutils.sysconfig'

	GLOB sdist-make: /home/kirr/src/tools/go/pygolang/setup.py
	py36-thread create: /home/kirr/src/tools/go/pygolang/.tox/py36-thread
	ERROR: invocation failed (exit code 1), logfile: /home/kirr/src/tools/go/pygolang/.tox/py36-thread/log/py36-thread-0.log
	ERROR: actionid: py36-thread
	msg: getenv
	cmdargs: '/usr/bin/python3 -m virtualenv --python /usr/bin/python3.6 py36-thread'

	Traceback (most recent call last):
	  File "/usr/lib/python3/dist-packages/virtualenv.py", line 25, in <module>
	    import distutils.sysconfig
	ModuleNotFoundError: No module named 'distutils.sysconfig'
	Running virtualenv with interpreter /usr/bin/python3.6

	ERROR: Error creating virtualenv. Note that some special characters (e.g. ':' and unicode symbols) in paths are not supported by virtualenv. Error details: InvocationError('/usr/bin/python3 -m virtualenv --python /usr/bin/python3.6 py36-thread (see /home/kirr/src/tools/go/pygolang/.tox/py36-thread/log/py36-thread-0.log)', 1)

https://bugs.debian.org/918881#19

tox.ini

 [tox]
-envlist = {py27,py36,py37,pypy,pypy3}-{thread,gevent}
+envlist = {py27,py37,pypy,pypy3}-{thread,gevent}
 
 [testenv]
 deps    =