Use Cython < 3.1

Avoid:
>       breezy/bzr/_chk_map_pyx.pyx:43:0: 'cpython/int.pxd' not found
>       breezy/bzr/_chk_map_pyx.pyx:43:0: 'cpython/int/PyInt_AsUnsignedLongMask.pxd' not found

Author: Flamefire

.github/workflows/synchronize-launchpad.yml

@@ -24,7 +24,9 @@ jobs:
           . $venv_dir/bin/activate
           echo "VIRTUAL_ENV=$VIRTUAL_ENV" >> "$GITHUB_ENV"
           echo "$venv_dir/bin" >> "$GITHUB_PATH"
-          pip install Cython configobj 'pip<23.1'  # Use a pip version that still allows the legacy installation for this breezy version
+          # Use a pip version that still allows the legacy installation for this breezy version
+          # Use a Cython still having e.g. cpython/int.pxd (Python 2.x legacy)
+          pip install 'pip<23.1' 'Cython<3.1' configobj
           pip install 'breezy[fastimport,git,launchpad]<3.3'  # 3.3+ requires interactive login
           fastimport_dir=$venv_dir/lib/python*/site-packages/breezy/plugins/fastimport
           # Add missing import and use binary buffer of stdin for Python3 compat