chore: automate dep updates

[kbdharun]

Changes

This PR adds a dependabot.yml config file to automate dependency updates for the GitHub actions (version) and the Python requirements.txt file.

Dependabot is a built-in security feature in GitHub, it automatically opens PRs to update dependencies if there are moderate or high severity CVEs attached to an action/dependency (if it is enabled in https://github.com/geigi/cozy/security), this PR adds support for normal version updates (monthly) using dependabot for the dependencies. (In future, manual updates like #808 aren't necessary for the actions)

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Offtopic: I noticed there are travis CI files in the repo, is it still being used (now that testing and releases are done via GitHub actions).

[rdbende]

Thanks! This looks good to me, but I'll leave it to @geigi, he knows the current CI setup better.

[rdbende]

Would be so cool if Dependabot could do update the Pypi dependencies in the Flatpak manifest as well.

[kbdharun]

Would be so cool if Dependabot could do update the Pypi dependencies in the Flatpak manifest as well.

Dependabot, unfortunately, doesn't support updating dependencies in the Flatpak manifest. I think https://github.com/flathub-infra/flatpak-external-data-checker would work for your use case.

[geigi]

Thanks for your contribution :) Travis CI is not used anymore so probably some leftover stuff from long time ago.