Update ixmp4 requirement to >=0.7.1

[phackstock]

Please confirm that this PR has done the following:

• [ ] Tests Added
• [ ] Documentation Added
• [x] Name of contributors Added to AUTHORS.rst
• [x] Description in RELEASE_NOTES.md Added

Description of PR

This PR moves the ixmp4 version requirement to >=0.7.1. Thanks a lot @meksor and @glatterf42 for working on the ixmp4 fix.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.8%. Comparing base (3d13de4) to head (48e5b47).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #824 +/- ## ===================================== Coverage 94.8% 94.8% ===================================== Files 64 64 Lines 6089 6089 ===================================== Hits 5775 5775 Misses 314 314 ```

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