tests: cleanup and ci hardening (#2397)

* tests: refactor and cleanup

* refactor: more consistent

* tests: vendor six

* tests: more xfails, nicer system

* tests: simplify to info

* tests: suggestions from @YannickJadoul and @bstaletic

* tests: restore some pypy tests that now pass

* tests: rename info to env

* tests: strict False/True

* tests: drop explicit strict=True again

* tests: reduce minimum PyTest to 3.1

tests/env.py

@@ -0,0 +1,12 @@
+# -*- coding: utf-8 -*-
+import platform
+import sys
+
+LINUX = sys.platform.startswith("linux")
+MACOS = sys.platform.startswith("darwin")
+WIN = sys.platform.startswith("win32") or sys.platform.startswith("cygwin")
+
+CPYTHON = platform.python_implementation() == "CPython"
+PYPY = platform.python_implementation() == "PyPy"
+
+PY2 = sys.version_info.major == 2