* Move `pympler` back into the `all` extras

Undoes a change I made in #12381. I can't fully remember my reasoning,
but this changed the contents of the debian packages in a backwards
incompatible way. We're not aware of anyone who's been bitten by this,
but we still want to fix it.

To the reviewer: please be convinced that the debian packages will still
contain pympler after this change.

* Debian changelog entry to keep the linter happy

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

This seems to make dch happy when we prepare the release.

* Use `poetry` to build venv in debian packages

Co-authored-by: Dan Callahan <danc@element.io>
Co-authored-by: Shay <hillerys@element.io>

* Changelog

* Only pull in from requirements.txt

Addresses the same problem as #12439.

* Include `test` and `all` extras

`poetry export` helpfully silently ignores an unknown extra

Haven't seen this before because it's the only place we export `all` and
`test`. I could have __sworm__ that the syntax `--extra "all test"`
worked for `poetry install`...

* Clean up requirements file on subsequence builds

* Fix shell syntax

Co-authored-by: Dan Callahan <danc@element.io>
Co-authored-by: Shay <hillerys@element.io>