Update the project for 2020

- replace pipenv with poetry
- update the README.md:
  * streamline the text
  * update links to notebooks with nbviewer
  * update installation notes with poetry info
- streamline the notebooks:
  * use backticks in MarkDown cells to make references to
    columns in DataFrames clearer
  * blacken all code cells
- add MIT license
- ignore .venv/ and .python-version