Fix dependency on ART

[mzweilin]

ART v1.15.1 and v1.15.2 fix a few bugs since v1.15.0, but armory-testbed[engine] freezes the dependency on ART v1.15.0.

I would recommend using tilde requirements (https://python-poetry.org/docs/dependency-specification/) in case of future bug fixes in ART v1.15.*

"adversarial-robustness-toolbox ~= 1.15.0"

https://github.com/twosixlabs/armory/blob/fd853f2910a5b0c8d5bf75bad22ff2b1800f09d8/pyproject.toml#L64C37-L64C38

[swsuggs]

Thanks Weilin. We typically pin ART at each release so we can be sure everyone is using the same version, allowing consistency between evaluators. The eval8 release will use ART 1.16.0, see #1999