Replace pipenv by poetry

[cbartz]

Reasons

• multiple Python Versions can be specified
• poetry uses standard-based (PEP 518) pyproject.toml file
• poetry allows easy building and pushing to PyPI

[codecov-commenter]

Codecov Report

Base: 93.64% // Head: 93.64% // No change to project coverage 👍

Coverage data is based on head (dc60b0f) compared to base (34248b7).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master       #5   +/-   ##
=======================================
  Coverage   93.64%   93.64%           
=======================================
  Files          20       20           
  Lines         708      708           
=======================================
  Hits          663      663           
  Misses         45       45           

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