[self-tests] fail against packaging 26.1

With recently released `packaging 26.1` (https://packaging.pypa.io/en/latest/changelog.html):
```
_____________________________________________ [doctest] jaraco.versioning.SummableVersion ______________________________________________
051
052 A special version that can be added to another Version.
053
054 >>> SummableVersion('1.1') + packaging.version.Version('2.3')
Expected:

Got:

/usr/src/RPM/BUILD/jaraco.versioning/jaraco/versioning.py:54: DocTestFailure
```

Start with the doctest for SummableVersion in jaraco/versioning.py at line 54 and compare its behavior with packaging 26.1. Run the self-tests against that dependency version and inspect the failure to determine the expected compatibility behavior. Done means the relevant doctest passes with packaging 26.1 without breaking the existing versioning behavior.

Written by the indexing model from the issue text.