Don't break --failed-first when re-ordering tests (#820)

Pytest-django 3.5.0 attempts to run tests in the same order as Django:
Issue #223, Commit 5a79fba

Pytest itself can re-order tests to prioritize previously failed tests
(--failed-first), incremental runs of the test suite (--stepwise), etc.
However, pytest-django's re-ordering clobbers pytest's, which can break
those options as reported in #819.

By applying the @pytest.hookimpl(tryfirst=True) decorator to
pytest_collection_modifyitems(), pytest-django's re-ordering happens
first, and does not clobber the later re-ordering performed by pytest.

Fixes #819

Author: callahad

pytest_django/plugin.py

@@ -415,6 +415,7 @@ def pytest_runtest_setup(item):
             _disable_class_methods(item.cls)
 
 
+@pytest.hookimpl(tryfirst=True)
 def pytest_collection_modifyitems(items):
     def get_order_number(test):
         marker_db = test.get_closest_marker('django_db')