Example of my installation

Installation path of python in my computer is

C:\Python36-32

So in control panel in system settings in advanced settings in path i add the following paths

C:\Python36-32
C:\Python36-32\Scripts

Now i reopen the cmd and i enter python. If it worked the python interpreter line opens.

To install flake8 system-wide run the command

pip install flake8

In atom editor install the package linter-flake8.When package is installed in settings of package in path add the full path of package for example in my configuration i have in path:

C:\Python36-32\Scripts\flake8   without the .exe