Version 1.2.0 — Refresh for 2023

Some maintenance updates:

• The package now uses a pyproject.toml file instead of setup.rb for packaging, per PEP 621.
• Requirements.txt has been accordingly deleted.
• Python 3.6 has been removed from the project classifiers and test grid.
• Build and publish scripts on Github have been accordingly updated.

Full Changelog: v1.1.5...v1.2.0