Use poetry-core as build-system

[rytilahti]

Using poetry-core allows performing installation without having full poetry setup installed, e.g.:

$ pip install .

should work now directly.

[codecov-commenter]

Codecov Report

Merging #1152 (887b02b) into master (43d9eaf) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1152   +/-   ##
=======================================
  Coverage   76.06%   76.06%           
=======================================
  Files          76       76           
  Lines        8921     8921           
  Branches      748      748           
=======================================
  Hits         6786     6786           
  Misses       1951     1951           
  Partials      184      184           

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