WIP: use hookwrapper to handle items in the end

This allows for proper grouping when using e.g. `-k`: items will be
narrowed down already by then.

TODO:
 - [ ] will throw an "Invalid test-group argument" when `-k` selects
   only a single test and you use test-group-count=2 test-group=2.

pytest_test_groups/__init__.py

@@ -1,4 +1,7 @@
 import math
+from random import Random
+
+import pytest
 
 from random import Random
 
@@ -27,7 +30,9 @@ def pytest_addoption(parser):
                     help='Integer to seed pseudo-random test ordering')
 
 
+@pytest.hookimpl(hookwrapper=True)
 def pytest_collection_modifyitems(session, config, items):
+    yield
     group_count = config.getoption('test-group-count')
     group_id = config.getoption('test-group')
     seed = config.getoption('random-seed', False)