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/pybind11_tests.cpp

@@ -88,6 +88,4 @@ PYBIND11_MODULE(pybind11_tests, m) {
 
     for (const auto &initializer : initializers())
         initializer(m);
-
-    if (!py::hasattr(m, "have_eigen")) m.attr("have_eigen") = false;
 }