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/pytest.ini

@@ -1,11 +1,14 @@
 [pytest]
-minversion = 3.0
+minversion = 3.1
 norecursedirs = test_cmake_build test_embed
+xfail_strict = True
 addopts =
     # show summary of skipped tests
     -rs
     # capture only Python print and C++ py::print, but not C output (low-level Python errors)
     --capture=sys
+    # enable all warnings
+    -Wa
 filterwarnings =
     # make warnings into errors but ignore certain third-party extension issues
     error