tox: run tests for python 3

also simplify a bit definition as ZODB is common in all versions

----

kirr:

- cover only last 2 py3 releases: 3.6 and 3.7 for now (3.8 is not yet
  released)
- separate ZODB3 as it supports only python2.

Py3 tests are failing for now and we'll be getting them to pass
incrementally - step by step.

tox.ini

 # zodbtools | tox setup
 [tox]
-envlist = py27-{ZODB3,ZODB4,ZODB5}
+envlist = py27-ZODB3, py{27,36,37}-ZODB{4,5}
 
 [testenv]
 deps	=