attempt to make keep an extra to put it on the PATH when installing via pipx/homebrew

For anyone who might have time to take up the mantle of keeping keep on the PATH here (see #425 for context):

I missed a quote. Let's pretend it was intentional to keep this rush-job from getting merged in ;) I have no experience with setuptools and unfortunately don't have more time to tinker- just stumbled across this potential solution and thought I should share

Of particular interest in #425 thread, last comment: as mentioned, consider use case where user opts not to install keep.

• Bug fixes

How to test:

I think you should be able to clone this branch, fix the quote, and pipx install .[keep]

Not sure how to test w/ brew

@gleitz feel free to close if this just adds noise to your repo i wont be offended :)

Please provide detailed instructions for testing your changes locally, including expected response/behavior.

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