add open-mastr user agent to bulk download requests

[maurerle]

Summary of the discussion

adds a User-Agent to the requests when doing a bulk download. The importlib.metadata is available in the python core since version 3.9, so for all supported versions.

In the rare but possible case, that the open-mastr module is called directly without being installed in any way, the user-agent is simply set to open-mastr without any version.

As all requests also contain the version of the python-requests library, this is added as well to provide metadata.

Workflow checklist

Automation

Closes #489

PR-Assignee

• [ ] 🐙 Follow the workflow in CONTRIBUTING.md
• [ ] 📝 Update the CHANGELOG.md
• [ ] 📙 Update the documentation

Reviewer

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• [ ] 🐙 Provided feedback and show sufficient appreciation for the work done

[FlorianK13]

API Credentials are not revealed for PRs from outside. I will merge anyway and rely on the tests that are running before the next release.

Thanks @maurerle for your PRs!