CI run against Twisted trunk is failing

[github-actions]

See https://github.com/matrix-org/synapse/actions/runs/2200519360

[richvdh]

rerunning this to see if it was a transient

[DMRobertson]

  × pip subprocess to install build dependencies did not run successfully.
  │ exit code: 1
  ╰─> [5 lines of output]
      Looking in links: /pypi-offline-cache
      Processing /pypi-offline-cache/setuptools-60.10.0-py3-none-any.whl
      Processing /pypi-offline-cache/wheel-0.37.1-py2.py3-none-any.whl
      ERROR: Could not find a version that satisfies the requirement incremental>=21.3.0 (from versions: none)
      ERROR: No matching distribution found for incremental>=21.3.0
      [end of output]

One for @Squah?

(Though I think we've had successes for this job after #12425 landed?)

[squahtx]

It looks like we're successfully patching the lockfile to point at twisted trunk, and installing it.
The failure happens while trying to install synapse in offline (no internet) mode. pip decides to reinstall twisted trunk and complains because twisted declares incremental as a build dependency (https://github.com/twisted/twisted/blob/trunk/pyproject.toml#L5). Since we've disabled pypi (--no-index), pip can't find it.

We shouldn't be installing twisted trunk at this part of the process, since it was already installed by an earlier CI step. Perhaps pip isn't smart enough to recognise that twisted trunk is already installed?

Adding the --no-deps flag to pip ought to work. We'll still need --no-index, otherwise pip will want to use pypi for build deps. We'll want to run pip check afterwards to fail if there are unmet dependencies.

[squahtx]

I'm working on a fix for this...