Use versioning-strategy increase-if-necessary

[gadomski]

Description:

It's a little thing, but I think this will get it to respect ~= in our pyproject.toml, per https://github.com/dependabot/dependabot-core/issues/1440 and https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#versioning-strategy. No CHANGELOG entry required.

PR Checklist:

• [x] Code is formatted
• [x] Tests pass

[gadomski]

Could also consider changing the schedule to monthly if its feeling like too many PRs

This repo's not too bad (unlike stac-api-validator, sheesh)