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_plotting.py

@@ -1,15 +1,16 @@
 import json
-import numpy as np
-import xgboost as xgb
-import testing as tm
 
+import numpy as np
 import pytest
 
+import xgboost as xgb
+from xgboost import testing as tm
+
 try:
     import matplotlib
     matplotlib.use('Agg')
-    from matplotlib.axes import Axes
     from graphviz import Source
+    from matplotlib.axes import Axes
 except ImportError:
     pass