Easy swap of local dependency

[mwarkentin]

We are using devenv in ops now, and recently sentry-kube has been extracted from that repo and into a standalone one: https://github.com/getsentry/sentry-infra-tools/

Sometimes we need to work on sentry-kube itself, and it's nice to iterate on it locally. Now that it's installed externally, we need to switch to a local install with -e. We can do that manually by modifying requirements.txt and pip install -r requirements.txt (see docs), but it would be nice if there was some cleaner way to do this within devenv that doesn't leave us out of sync with the checked in code, etc.

Not sure if there's a nice way to do this, but worth a shot. :)