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_virtual_functions.py

@@ -1,6 +1,8 @@
 # -*- coding: utf-8 -*-
 import pytest
 
+import env  # noqa: F401
+
 from pybind11_tests import virtual_functions as m
 from pybind11_tests import ConstructorStats
 
@@ -160,7 +162,7 @@ def test_alias_delay_initialization2(capture):
 
 # PyPy: Reference count > 1 causes call with noncopyable instance
 # to fail in ncv1.print_nc()
-@pytest.unsupported_on_pypy
+@pytest.mark.xfail("env.PYPY")
 @pytest.mark.skipif(not hasattr(m, "NCVirt"), reason="NCVirt test broken on ICPC")
 def test_move_support():
     class NCVirtExt(m.NCVirt):