Fix skipping test_config_initialization for pytest<5

Fix test_config_initialization to be correctly skipped on pytest<5,
by explicitly checking pytest version (idea copied from
test_looponfail_removed_test).  The current conditions are insufficient
-- the outer check wrongly assumes pytest>=5 will always be used
on Python 3 (which is not true if you need the same version to support
both Python 2 and Python 3), and the inner condition apparently
wrongly assuming that invocation_params attribute is not present
in pytest-4.

changelog/566.bugfix

@@ -0,0 +1 @@
+Fix test_config_initialization not being skipped correctly on pytest-4