Move `pympler` back into the `all` extras (#12652)

* 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