Add a Dependabot config to autoupdate GitHub action versions

[kurtmckee]

This PR adds a Dependabot configuration to auto-update GitHub action versions.

Currently, most of the action versions are up-to-date since they were introduced. However, actions/[email protected] action has been updated several times, including various fixes and improvements (it is currently at v3.3.1).

Rather than submitting a PR to update this as a one-off, I'm submitting a PR to configure Dependabot to submit PRs that will keep the action versions evergreen.

If this merges, you can anticipate that Dependabot will immediately submit a PR to update actions/cache to the latest version.

As this creates a new file and doesn't change code, documentation, tests, or even directly change CI, I haven't filled out the checkboxes below. I also didn't create an issue as this didn't seem to fit the templated issue reports for bug fixes nor feature requests. However, please let me know if I'm mistaken and need to change anything to make this PR ready for review.

Thanks!

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Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[pgjones]

Thanks, I've merged manually as I pressed the wrong button.