fix pytest_ignore_collect not to block default pytest code

Fix `pytest_ignore_collect` hook implementation to return `None`
rather than `False` when the path ought not to be ignored.  This is
necessary to enable the default pytest implementation of
`pytest_ignore_collect()` to be used.  Otherwise, the default rules
are never applied and e.g. `--ignore` does not work.

I've also reported pytest-dev/pytest#12383
about the misleading documentation.

conftest.py

@@ -19,4 +19,4 @@ def pytest_ignore_collect(collection_path):
     """Text collection function executed by pytest"""
     if not WITH_ASYNC and basename(str(collection_path)) in async_files:
         return True
-    return False
+    return None