Filter warnings on pytest >= v3.1

The new version of pytest now reports Python warnings by default. This
commit filters out some third-party extension warnings which are not
useful for pybind11 tests.

tests/pytest.ini

@@ -5,3 +5,10 @@ addopts =
     -rs
     # capture only Python print and C++ py::print, but not C output (low-level Python errors)
     --capture=sys
+filterwarnings =
+    # make warnings into errors but ignore certain third-party extension issues
+    error
+    # importing scipy submodules on some version of Python
+    ignore::ImportWarning
+    # bogus numpy ABI warning (see numpy/#432)
+    ignore:.*numpy.dtype size changed.*:RuntimeWarning