Add OpenSSF Best Practices Badge, Address Warnings

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• (If applicable) Documentation has been added / updated (for bug fixes / features).
• (If applicable) Tests have been added.
• CHANGELOG.rst has been updated (with summary of main changes).
  • Link to issue (:issue:number) and pull request (:pull:number) has been added.

What kind of change does this PR introduce?

• Adds the OpenSSF Best Practices badge (compliance is at 100%!)
• Addresses several warnings from deprecated usages.

Does this PR introduce a breaking change?

It should not.

Other information:

https://www.bestpractices.dev/en/projects/10064

[coveralls]

coverage: 82.084% (+0.03%) from 82.055%
when pulling e2d9247 on ossf-bp
into 97e5163 on master.