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

pyproject.toml

@@ -0,0 +1,37 @@
+[build-system]
+build-backend = "poetry.masonry.api"
+requires = ["poetry>=0.12"]
+
+[tool.poetry]
+name = "tidy-data"
+version = "0.1.0"
+
+authors = ["Alexander Hess <alexander@webartifex.biz>"]
+description = "A Python implementation for Hadley Wickham's Tidy Data paper"
+keywords = [
+    "data-cleaning",
+    "data-science",
+    "messy-data",
+    "python",
+    "tidy-data",
+]
+license = "MIT"
+
+[tool.poetry.dependencies]
+python = "^3.7"
+
+# Data Science Tools
+jupyterlab = "^2.2.6"
+matplotlib = "^3.3.1"
+numpy = "^1.19.1"
+pandas = "^1.1.1"
+seaborn = "^0.10.1"
+sklearn = "^0.0"
+
+# Interfaces to other tools
+rpy2 = "==2.8.*"  # R support
+savreaderwriter = "^3.4.2"  # IBM SPSS support
+
+# Code Formatters
+black = "^19.10b0"
+nb_black = "^1.0.7"