Pin poetry to specific version

The range existed because newer versions of `poetry` had dropped
`python` `3.6` support.

Now that we've dropped `3.6` / `3.7`, we can go back to pinning to a
specific version for more deterministic builds.

python/helpers/requirements.txt

@@ -3,7 +3,7 @@ pip-tools>=6.4.0,<=6.14.0  # Range maintains py36 support TODO: Review python 3.
 hashin==0.17.0
 pipenv==2022.4.8
 pipfile==0.0.2
-poetry>=1.1.15,<1.6.0
+poetry==1.5.1
 # For now we chose to pin `wheel` even though we don't import it directly.
 # Background context: https://github.com/dependabot/dependabot-core/pull/5597
 wheel==0.37.1