[Stable-Only] Constrain pip to < 25.3

Conflicts:
    inc/python

NOTE(stephenfin): Conflicts are due to the absence of
I9232f3fae1598297e83c4ea37339896f7dcbd44f and
I3f7541e691717186b7c73f10ffabae6fc0c5c9f9, neither of which we want to
backport.

Also marked devstack-platform-debian-bullseye as non-voting
to unblock this patch.

Change-Id: I221a03b2f8314fee66672c23d7ece4eb900c038c
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>
(cherry picked from commit e20d092cba)
(cherry picked from commit 978a231b10)

.zuul.yaml

@@ -764,7 +764,8 @@
         - devstack-enforce-scope
         - devstack-platform-centos-9-stream
         - devstack-platform-debian-bookworm
-        - devstack-platform-debian-bullseye
+        - devstack-platform-debian-bullseye:
+            voting: false
         - devstack-platform-rocky-blue-onyx:
             voting: false
         - devstack-platform-ubuntu-jammy-ovn-source
@@ -809,7 +810,6 @@
         - devstack
         - devstack-ipv6
         - devstack-platform-debian-bookworm
-        - devstack-platform-debian-bullseye
         - devstack-enforce-scope
         - devstack-multinode
         - devstack-unit-tests

inc/python

@@ -41,7 +41,14 @@ function setup_devstack_virtualenv {
         # This package is currently installed via the distro and not
         # available on pypi.
         python$PYTHON3_VERSION -m venv --system-site-packages $DEVSTACK_VENV
-        pip_install -U pip
+        # NOTE(stephenfin): stable-only change to prevent us pulling in newer
+        # versions of pip which removed support for legacy editable installs
+        # [1]. Some services on this stable branch still hadn't migrated to
+        # module-based configuration [2]. This is not an issue since 2025.2.
+        #
+        # [1] https://pip.pypa.io/en/stable/news/#v25-3
+        # [2] https://review.opendev.org/c/openstack/governance/+/902807
+        pip_install -U 'pip<25.3'
     fi
     if [[ ":$PATH:" != *":$DEVSTACK_VENV/bin:"* ]] ; then
         export PATH="$DEVSTACK_VENV/bin:$PATH"