Add upstream build tests

[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.
• This PR does not seem to break the templates.
• CHANGES.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 a dedicated upstream build test workflow that runs nightly to test for incompatibilities with upstream development dependencies
• Added a new requirements-upstream.txt file for installing development branch dependencies

Does this PR introduce a breaking change?

No.

Other information:

This approach was directly adapted from xclim.