I recently updated one of the packages in Atom that was called linter-pep8 to version 2.0 which was renamed to linter-pycodestyle. This is because PEP8 the package was renamed to pycodestyle to reduse confusion between the package and the PEP8 the specification.

However after I opened Atom I got an error message Error: spawn pycodestyle ENOENT

because I hadn't upgraded the python package. As I wasn't using PEP8 for anything else I uninstalled it and installed pycodestyle. On windows I'd installed Python 3.6 x64 for all users so it was Python was installed C:\Program Files\Python36\

"C:\Program Files\Python36\Scripts\pip.exe" uninstall pep8
"C:\Program Files\Python36\Scripts\pip.exe" install pycodestyle

on Linux¹ pip was in my PATH environment varable so I simply ran

sudo pip uninstall pep8
sudo pip install pycodestyle

And that fixed up my issues.