Fix broken Python 2 build

[bessbd]

Checklist

• [X] Tick to sign-off your agreement to the Developer Certificate of Origin (DCO) 1.1
• [ ] Added tests for code changes or test/build only changes
• [X] Updated the change log file (CHANGES.md|CHANGELOG.md) or test/build only changes
• [X] Completed the PR template below:

Description

f2c1298 accidentally broke the build as collections.abc is only available in Python 3.x.

This commit adds a workaround that enables the import that works both on 2 and 3.

Approach

Schema & API Changes

"No change"

Security and Privacy

Testing

No new tests because the change is not semantic.

Monitoring and Logging

"No change"

[bessbd]

@ricellis , @emlaver : Thank you for the review! I'm about to merge this PR.