howto: New guide Add Black and isort to an existing Python repository

[ghost]

#80

[ghost]

Do merge if happy

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Python 3.6.9 (default, Nov  7 2019, 10:44:02) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.version_info >= (3, 6)
True
>>> sys.version_info >= (3, 5)
True
>>> sys.version_info >= (3, 7)
False
>>> sys.version_info >= (4, 0)
False
>>> (4,0) >= (3,6)
True
>>> (3,7) >= (3,6)
True
>>> (3,5) >= (3,6)
False