Move Python testing utilities into xgboost module. (#8379)

- Add typehints.
- Fixes for pylint.

Co-authored-by: Hyunsu Philip Cho <chohyu01@cs.washington.edu>

tests/python/test_interaction_constraints.py

@@ -1,9 +1,9 @@
-# -*- coding: utf-8 -*-
 import numpy as np
-import xgboost
-import testing as tm
 import pytest
 
+import xgboost
+from xgboost import testing as tm
+
 dpath = 'demo/data/'
 rng = np.random.RandomState(1994)