[MXNET-696][PYTHON][UNDEFINED NAME] import logging in ci/util.py (#1…

…2488)

Fixes #12406 @larroy @marcoabreu @szha 

[flake8](http://flake8.pycqa.org) testing of https://github.com/apache/incubator-mxnet on Python 3.7.0

$ __flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics__
```
./ci/util.py:73:21: F821 undefined name 'logging'
                    logging.warning("Exception: %s, Retrying in %d seconds...", str(e), mdelay)
                    ^
1     F821 undefined name 'logging'
1
```

ci/util.py

@@ -17,6 +17,7 @@
 
 import os
 import contextlib
+import logging
 import requests
 
 def get_mxnet_root() -> str: