Add deprecation notice for NamespacePackageEncountered

[nathanjmcdougall]

[x] Add tests for the change. In general, aim for full test coverage at the Python level. Rust tests are optional.
[ ] Add any appropriate documentation.
[x] Add a summary of changes to CHANGELOG.rst.
[x] Add your name to AUTHORS.rst.
[x] Run just full-check.

Resolves #272

[codspeed-hq]

CodSpeed Performance Report

Merging #276 will not alter performance

_{Comparing nathanjmcdougall:maint/add-deprecation-for-NamespacePackageEncountered (f996d29) with main (f397742)}

Summary

✅ 26 untouched
⏩ 23 skipped¹

Footnotes

1. 23 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[seddonym]

Nice job!

Couple of minor suggestions before we merge.

(Also you probably want to rebase off the latest main branch as there's a conflict.)