[PATCH 0 of 1] db: link relationship pairs that share a foreign key

[橋本 憲一]

SQLAlchemy 1.4 prints 35 relationship overlap warnings when the models are
configured. This patch links each pair with back_populates so they go away.
It also fixes a hang of the Hg ssh tests on 1.4; the commit message has the
details.

Tested with the full test suite on stable 4c5d75319b9e, Python 3.11.15,
NetBSD 11.0:

                       stable                  with this patch
  SQLAlchemy 1.4.54    35 warnings             0 warnings
                       ssh tests hang          ssh tests pass
                       (20 tests deselected)
  SQLAlchemy 1.3.24    0 warnings              0 warnings

1.3 does not emit these warnings, so it is there to check that nothing
changes. I did not test 1.2.

In all four runs, the same 14 tests failed and the same 3 tests had
errors, with and without the patch. Two of the failures are the
markup_renderer doctests, which fail with bleach 6 (I sent a separate
patch for that). The others are 500 responses and fixture errors in the
summary, fork and admin repo tests. They fail the same way on unpatched
stable in this environment.