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/generate_models.py

@@ -1,7 +1,9 @@
-import xgboost
-import numpy as np
 import os
 
+import numpy as np
+
+import xgboost
+
 kRounds = 2
 kRows = 1000
 kCols = 4