Improve CI: drop testing pytest master on py27, use py37

* Use Python 3.7 as preferred Python 3 interpreter
* Do not test pytest `master` and `features` branches on
  Python 2.7, as those branches no longer support Python 2.7/3.4

tox.ini

@@ -2,7 +2,7 @@
 envlist=
   linting
   py{27,34,35,36,37}-pytestlatest
-  py{27,36}-pytest{master,features}
+  py37-pytest{master,features}
 
 [testenv]
 passenv = USER USERNAME
@@ -24,7 +24,7 @@ commands = pre-commit run --all-files --show-diff-on-failure
 [testenv:release]
 changedir=
 decription = do a release, required posarg of the version number
-basepython = python3.6
+basepython = python3.7
 skipsdist = True
 usedevelop = True
 passenv = *