fix: add six dependency — undeclared transitive dep of python-pam (breaks macOS clean envs)

python-pam imports six but doesn't declare it. On Linux, six is usually
present system-wide. On macOS uv tool environments (clean/isolated),
import pam fails with 'No module named six'. Adding six explicitly to
our deps ensures PAM auth works on macOS.

pyproject.toml

@@ -15,6 +15,7 @@ dependencies = [
     "aiofiles>=23.0",
     "websockets>=11.0",
     "python-pam>=1.8.4",
+    "six>=1.16.0",              # undeclared dep of python-pam (needed on macOS clean envs)
     "itsdangerous>=2.1.0",
     "python-multipart>=0.0.9",
 ]