ci: Enable Dependabot version updates for GitHub Actions

[henryiii]

Description

This would help keep actions up to date.

c.f. https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Add GitHub Dependabot config to check for GitHub Action version updates and
to open PRs if available. Set the check and update interval to weekly.
   - c.f. https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot

[codecov]

Codecov Report

Merging #1900 (df656be) into master (cac88cd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1900   +/-   ##
=======================================
  Coverage   98.17%   98.17%           
=======================================
  Files          68       68           
  Lines        4331     4331           
  Branches      730      730           
=======================================
  Hits         4252     4252           
  Misses         46       46           
  Partials       33       33           

Flag	Coverage Δ
contrib	26.34% <ø> (ø)
doctest	60.63% <ø> (ø)
unittests-3.10	96.05% <ø> (ø)
unittests-3.7	96.03% <ø> (ø)

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[matthewfeickert]

Thanks @henryiii. I had been meaning to look into how much of dependabot we needed to opt into for this to work, but I guess the answer is nothing more than this.

[matthewfeickert]

LGTM. 👍 Thanks @henryiii!