[securedrop-proxy] Switch this package to poetry for Python dependency management

[eloquence]

• Add support for Poetry in securedrop-builder - Add support for building packages that use Poetry securedrop-builder#467
• Update build-requirements.txt path in a backwards-compatible manner Update build-requirements.txt path for securedrop-proxy package securedrop-builder#469
• Re-run CI on Use Poetry for dependency management #122 and ensure expected checks are green (PyYAML errors on Bookworm are expected, see [securedrop-proxy] bookworm CI is failing because of pyyaml incompatibility with cython 3 securedrop-client#1681)
• Review and merge Use Poetry for dependency management #122
• Re-run CI on Finalize proxy path securedrop-builder#470 and ensure expected checks are green
• Review and merge Finalize proxy path securedrop-builder#470

[eloquence]

@legoktm Proposed order of operations for switching proxy over :)

[legoktm]

All done \o/