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/test_async.py

@@ -1,7 +1,8 @@
 # -*- coding: utf-8 -*-
-import asyncio
 import pytest
-from pybind11_tests import async_module as m
+
+asyncio = pytest.importorskip("asyncio")
+m = pytest.importorskip("pybind11_tests.async_module")
 
 
 @pytest.fixture