Update poetry to use core back-end

[bachya]

Describe what the PR does:

This PR ensures that poetry uses the split-out core back-end during building.

Does this fix a specific issue?

N/A

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Update README.md with any new documentation.
• Add yourself to AUTHORS.md.

[codeclimate]

Code Climate has analyzed commit 567b353 and detected 0 issues on this pull request.

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[codecov]

Codecov Report

Merging #47 (567b353) into dev (c9f9898) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               dev       #47   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines          144       144           
=========================================
  Hits           144       144           

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