Don't specify transitive dependencies in admin/requirements.in

[rocodes]

Description

Following up on (#7116 (comment))

It looks like https://pip.pypa.io/en/latest/user_guide/#constraints-files is the way to handle this, but needs a little more looking.

[cfm]

See also freedomofpress/securedrop-client#1538.

[deeplow]

Not sure I get the full context, but was this fixed by any chance with #7551 and other constraints-related commits.