Fix up requirements.txt files used for boostrapping

[legoktm]

Description

They're kind of split up weirdly right now. For example, docker-requirements.in is:

pip>=21.3
setuptools>=56.0.0
wheel

But in the Dockerfile we do:

    /opt/venvs/securedrop-app-code/bin/pip3 install --no-deps --require-hashes -r requirements/python3/docker-requirements.txt && \
    /opt/venvs/securedrop-app-code/bin/pip3 install --no-deps --require-hashes -r requirements/python3/test-requirements.txt && \
    /opt/venvs/securedrop-app-code/bin/pip3 install --no-deps --require-hashes -r requirements/python3/securedrop-app-code-requirements.txt 

It probably should be named bootstrap-requirements.txt or something.

Then we have translation-requirements.in, which is:

setuptools-scm>=6.0.1

Another bootstrapping thing, you'd expect to find things like pybabel in translation-requirements, but nope.

[legoktm]

My specific proposal is bootstrap-requirements.in:

pip>=21.3
setuptools>=56.0.0
setuptools-scm>=6.0.1
wheel

And then translation-requirements.in:

babel>=2.9.1
sh

(sh might go away in https://github.com/freedomofpress/securedrop/issues/6547). Waiting on #6544 because it'll probably conflict with that.