Add a config for GitHub Dependabot

[webknjaz]

Dependabot has been acquired by GitHub a while back. And now they are migrating over from the dependabot-preview GitHub App to a built-in native GitHub Dependabot. It'll send pin updates the same way but you'll also be able to see the logs @ https://github.com/python-trio/snekomatic/network/updates.

This PR adds an initial config necessary for this migration.

The config and extra docs can be found here: https://docs.github.com/en/github/administering-a-repository/configuration-options-for-dependency-updates.

[codecov[bot]]

Codecov Report

Merging #114 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #114   +/-   ##
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  Coverage   94.39%   94.39%           
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  Files          13       13           
  Lines         660      660           
  Branches       56       56           
=======================================
  Hits          623      623           
  Misses         28       28           
  Partials        9        9