[MXNET-696] Define cmp() in Python 3 again

[cclauss]

Description

A second attempt at #12191 with no Unicode characters this time...

Fixes #8270 @vandanavk

cmp() was removed in Python 3 so this PR recreates it if required leveraging the formula at http://python-future.org/compatible_idioms.html#cmp

Description

Define cmp() in Python 3

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [MXNET-$JIRA_ID], where $JIRA_ID refers to the relevant JIRA issue created (except PRs with tiny changes)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$PR_ID/$BUILD_ID/index.html
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

• Define cmp() in Python 3

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

@marcoabreu #12237 (comment)

[vandanavk]

@piyushghai

[piyushghai]

LGTM.
Thanks for fixing the pylint issue here.

[lebeg]

cmp = lambda(x, y): (x > y) - (x < y) could also work with a bit less code

[cclauss]

1. It is a Python 3 syntax error
2. With the syntax fixed it generates a PEP8 violation in flake8 https://stackoverflow.com/questions/25010167/e731-do-not-assign-a-lambda-expression-use-a-def

[lebeg]

Ok, I see. Thanks!

[sandeep-krishnamurthy]

Thanks for your contributions!
This PR is ready to merge after CI pass.
I have restarted the PR build as it earlier failed with file download transient issue.