Configure black, flake8, and isort to work together

As recommended by Black:
https://github.com/psf/black/blob/master/docs/compatible_configs.md

.flake8

@@ -0,0 +1,4 @@
+[flake8]
+max-line-length = 88
+select = B,BLK,C90,E,F,I,W
+extend-ignore = E203, W503

pyproject.toml

@@ -26,6 +26,18 @@ pyls-mypy = "^0.1.8"
 python-language-server = "^0.34.1"
 rope = "^0.17.0"
 
+[tool.black]
+line-length = 88
+target-version = ['py38']
+
+[tool.isort]
+multi_line_output = 3
+include_trailing_comma = true
+force_grid_wrap = 0
+use_parentheses = true
+ensure_newline_before_comments = true
+line_length = 88
+
 [build-system]
 requires = ["poetry>=0.12"]
 build-backend = "poetry.masonry.api"