Migrate dependency management to poetry

[legoktm]

Switch dependency management to use poetry, which is much nicer than pip-tools. This is largely based off of
freedomofpress/securedrop-proxy#122 and applies the same changes to the Makefile and CI.

---

note: this will fail CI until freedomofpress/securedrop-builder#472 is merged.

[legoktm]

So for test plan, I think just verifying all the various make test, etc. commands you run locally during development work and then also doing a package build works.

I think going through the whole test plan in freedomofpress/securedrop-builder#467 is overkill largely because it was testing all the changes in -builder, which have already been reviewd.

[rocodes]

So far so good :)

• visual review
• poetry install succecssful, updated make targets successful
• building a package from tip of sd-builder works great
  I just kicked CI too so we should be seeing lots of 💚

edit: lastly

• adding a dependency in pyproject.toml and running poetry update updates build reqs and poetry.lock, and hashes match the same dependency being added in build-requirements.in and updating dependencies via pip-compile

[rocodes]

LGTM. Thanks for making this happen :) (Gonna hold off on merge til end of the day in case you want to sneak the poetry version update in, and we should probably update the dev docs and/or the sdw wiki once some more of these changes land, but 🥳)

[legoktm]

(lack of docs issue is freedomofpress/securedrop-dev-docs#103)