add isort and pyflakes tp pp.toml + config isort

pyproject.toml

@@ -39,6 +39,8 @@ Sphinx = "^4.3.2"
 pytest = "^6.2.5"
 mypy = "^0.930"
 numpydoc = "^1.3.1"
+pyflakes = "^2.4.0"
+isort = "^5.10.1"
 
 [build-system]
 requires = ["setuptools", "poetry-core>=1.0.0"]
@@ -53,3 +55,8 @@ extend-exclude = '''
 # in the root of the project.
 ^/foo.py  # exclude a file named foo.py in the root of the project (in addition to the defaults)
 '''
+[tool.isort]
+multi_line_output = 3
+line_length = 79
+include_trailing_comma = true
+