upgrades: Allow matrix's new dependency to be installed
matrix-synapse 1.47.0-2 requires python3-frozendict 1.2-3 or higher. This is only available in bullseye-backports, testing and unstable but not bullseye.
Allow python3-frozendict to be installed from buster-backports. Further there is no longer any need to allow packages from buster-backports (on the master branch). So, drop all those.
Tests:
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On stable/testing, without matrix-synapse and its dependencies installed, run matrix-synapse functional tests.
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On stable, with older version of matrix-synapse and python3-frozendict, run unattened-upgrade and notice that matrix-synapse upgrades to latest version.
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Without this patch, run service. Notice the contents of /etc/apt/preferences.d/51freedombox.pref. When this patch is applied and service is restarted, upgrades app is setup again and the files contents will change as expected.
Signed-off-by: Sunil Mohan Adapa sunil@medhas.org