fix __getattr__ conflict in prefect.docker.__init__

[zzstoatzz]

closes #16171

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Fix unittest.assertWarns() compatibility with module __getattr__ handling in prefect.docker.__init__

issue arises bc unittest's assertWarns uses getattr() with a default value to safely check for __warningregistry__ on modules

https://github.com/python/cpython/blob/e546876d83e1cc3f761e1eeca57f8a4533d06d00/Lib/unittest/case.py#L290-L294

Our __getattr__ implementation was raising ImportError, preventing Python from handling the default value case, so this PR changes it to raise AttributeError to follow attribute lookup protocol (like the other dynamic imports we have)

[codspeed-hq]

CodSpeed Performance Report

Merging #16172 will not alter performance

_{Comparing getattr-docker (d219d60) with main (f1649e9)}

Summary

✅ 3 untouched benchmarks

[desertaxle]

LGTM!