Switch to Poetry for build & dependency management (#114)

[declension]

• Remove Pipfile and Pipfile.lock
• Add pyproject.toml based on existing Pipfile
• Build and commit lock file
• Convert build script to use Poetry (and sed) to generate requirements.txt
• Update Circle steps to use Poetry not Pipenv

This closes #114.

[coveralls]

Pull Request Test Coverage Report for Build 191

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.193%

---

Totals
Change from base Build 189:	0.0%
Covered Lines:	972
Relevant Lines:	1043

---

💛 - Coveralls