chore(deps): update policyengine to 6.7.3

[policyengine]

Fixes #823

Summary

Update policyengine.py from 6.7.0 to 6.7.3 in the simulation API runtime.

The policyengine[models] requirement selects the runtime versions bundled by policyengine.py 6.7.3:

• policyengine-core: 3.33.0
• policyengine-us: 2.2.1
• policyengine-uk: 2.130.1
• spm-calculator: 1.0.0

The bundle also selects these certified data releases:

• US: populace-us-2024-spm-20260915
• UK: policyengine-uk-data-1.58.1

UK release identity:

• certified dataset SHA-256: 056e80fd8e9151d78f432a25cde863340baee81304c6216a37fb575ce8563784
• certificate digest (receipt v1): a9c1512399075dd4e16e0a73a2b4abb4f9d87cf8358011f5bc09c11434df1e36
• release dispatch verified against the installed wheel: true
• qualification: none recorded

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[MaxGhenis]

Superseded by #826, which updated policyengine from 6.7.0 straight to 6.7.5 and merged at 22:45Z on 10/18 (35857b2). The 6.7.4 updater run failed on the PyPI index race, and this 6.7.3 draft was never landed. Closed by the W4 ACS-repoint reviewer.